POLYMER SCIENCE
AND TECHNOLOGY
Robert O. Ebewele
Department of Chemical Engineering
University of Benin
Benin City, Nigeria
CRC Press
Boca Raton New York
Copyright 2000 by CRC Press LLC
Library of Congress Cataloging-in-Publication Data
Ebewele, Robert Oboigbaotor.
Polymer science and technology / Robert O. Ebewele.
p. cm.
Includes bibliographical references (p.
) and index.
ISBN 0-8493-8939-9 (alk. paper)
1. Polymerization. 2. Polymers. I. Title.
TP156.P6E24 1996
668.9--dc20
95-32995
CIP
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Copyright 2000 by CRC Press LLC
PREFACE
The book is divided into three parts. The first part covers polymer fundamentals. This includes a brief
discussion of the historical development of polymers, basic definitions and concepts, and an overview
of the basis for the various classifications of polymers. It also examines the requirements for polymer
formation from monomers and discusses polymer structure at three levels: primary, secondary, and
tertiary. The relationship between the structure of the monomers and properties of the resulting polymer
is highlighted. This section continues with a discussion of polymer modification techniques. Throughout
the discussion, emphasis is on the structure-property relationship and several examples are used to
illustrate this concept.
The second part deals with how polymers are prepared from monomers and the transformation of
polymers into useful everyday articles. It starts with a discussion of the various polymer preparation
methods with emphasis on reaction mechanisms and kinetics. The control of molecular weight through
appropriate manipulation of the stoichiometry of reactants and reaction conditions is consistently emphasized. This section continues with a discussion of polymer reaction engineering. Emphasis is on the
selection of the appropriate polymerization process and reactor to obtain optimal polymer properties.
The section terminates with a discussion of polymer additives and reinforcements and the various unit
operations in polymer processing. Here again, the primary focus is on how processing conditions affect
the properties of the part produced.
The third part of the book deals with the properties and applications of polymers. It starts with a
discussion of polymer solution properties through the mechanical properties of polymers and concludes
with an overview of the various applications of polymer materials solids. The viscoelastic nature of
polymers is also treated. This section also includes a discussion of polymer fracture. The effects of
various molecular and environmental factors on mechanical properties are examined.
The primary focus of the book is the ultimate property of the finished polymer product. Consequently,
the emphasis throughout the book is on how various stages involved in the production of the finished
product influence its properties. For example, which polymerization process will be preferable for a
given monomer? Having decided on the polymerization process, which type of reactor will give optimum
product properties? What is the best type of processing technique for a given polymer material? How
do processing conditions affect the properties of the part produced and which polymer material is most
suitable for a particular application? The book addresses the elements that must be considered to come
up with appropriate answers to these types of questions. The distinguishing features of the book are
intended to address certain problems associated with teaching an elementary course in polymers:
1. For a vast majority of introductory polymer courses, very frequently the instructor has to rely on several
textbooks to cover the basics of polymers as none of the existing textbooks discusses the required
materials satisfactorily. Most students find dealing with several textbooks in an introductory course
problematic. This book attempts to remedy this problem. A deliberate effort has been made to cover
most of the areas normally taught in such an introductory course. Indeed, these areas are typical of
existing texts. However, the approach and depth of coverage are different. The book presents various
aspects of polymer science and technology in a readily understandable way. Emphasis is on a basic,
qualitative understanding of the concepts rather than rote memorization or detailed mathematical
analysis. Description of experimental procedures employed in the characterization of polymers has
been either completely left out or minimized. I strongly believe that this approach will appeal to those
students who will be learning polymer science for the first time.
2. None of the existing texts has worked examples. It is my experience that students feel more comfortable
with and generally prefer textbooks that illustrate principles being discussed with examples. I have
followed this approach throughout the text. In addition, each chapter has review problems; answers are
provided in a Solutions Manual. Both the worked examples and the review problems are designed to
provide additional insight to the materials covered. The overall objective of this approach is to enhance
the reader’s understanding of the material and build his/her confidence. Emphasis throughout the book
is on structure-property relationship and both the worked examples and review problems reflect this
basic objective.
Robert O. Ebewele
Copyright 2000 by CRC Press LLC
ACKNOWLEDGMENT
In writing this book, I have had to rely on materials from various sources. These sources have been
compiled as references at the end of each chapter. While I express my profound gratitude to publishers
for permission to use their materials, I apologize for ideas and materials which I have inadvertently
failed to acknowledge. I certainly do not lay claim to these published concepts and ideas.
The skeletal framework for this book was initiated during my student days at the University of
Wisconsin, Madison and over the years, the material in the book has been constantly refined as it was
being developed for use by successive generations of undergraduate and graduate students at the Ahmadu
Bello University, Zaria, Nigeria. The final version of the book was written during my sabbatical leave
at the Department of Chemical Engineering, University of Wisconsin, Madison, and subsequently during
my leave of absence at the Forest Products Laboratory Madison, Wisconsin. I am grateful to the Ahmadu
Bello University, Zaria, the University of Wisconsin, Madison and the Forest Products Laboratory,
Madison for providing me unlimited access to their library materials and other facilities. Finally, I am
indebted to the late Prof. J. A. Koutsky of the University of Wisconsin, Madison; Dr. George E. Myers
and Mr. Bryan H. River, formerly of the Forest Products Laboratory, Madison; and a host of others for
reviewing various parts of this book. Your contributions have greatly improved the quality of the book.
I, however, take full responsibility for any lapses and errors that may be contained in the book.
Copyright 2000 by CRC Press LLC
TABLE OF CONTENTS
PART I: FUNDAMENTALS
Chapter One
Introduction
I. Historical Development
II. Basic Concepts and Definitions
III. Classification of Polymers
A. Natural vs. Synthetic
B. Polymer Structure
1. Linear, Branched, or Cross-Linked Ladder vs. Functionality,
2. Amorphous or Crystalline
3. Homopolymer or Copolymer
4. Fibers, Plastics, or Elastomers
C. Polymerization Mechanism
D. Thermal Behaviour
E. Preparative Technique
F. End Use
IV. Problems
References
Chapter Two
Polymerization Mechanisms
I. Introduction
II. Chain-Reaction Polymerization
A. Initiation
B. Propagation
C. Termination
D. Chain Transfer
E. Diene Polymerization
III. Ionic and Coordination Polymerizations
A. Cationic Polymerization
B. Anionic Polymerization
C. Coordination Polymerization
IV. Step-Growth Polymerization
A. Typical Step-Growth Polymerizations
1. Polyesters
2. Polycarbonates
3. Polyamides
4. Polyimides
5. Polybenzidazoles and Polybenzoxazoles
6. Aromatic Ladder Polymers
7. Formaldehyde Resins
8. Polyethers
9. Polysulfides
10.Polysulfones
V. Ring-Opening Polymerization
A. Poly(Propylene Oxide)
B. Epoxy Resins
C. Polycaprolactam (Nylon 6)
VI. Problems
References
Copyright 2000 by CRC Press LLC
Chapter Three
Chemical Bonding and Polymer Structure
I. Introduction
II. Chemical Bonding
A. The Ionic Bond
B. The Covalent Bond
C. Dipole Forces
D. Hydrogen Bond
E. Induction Forces.
F. van der Waals (Dispersion) Forces
III. Primary Structure
A. Polarity of Monomers
IV. Secondary Structure.
A. Configuration
1. Diene Polymerization
2. Tacticity
B. Conformation
C. Molecular Weight
V. Tertiary Structure
A. Secondary Bonding Forces (Cohesive Energy Density)
B. Crystalline and Amorphous Structure of Polymers
1. Crystallization Tendency
2. Structural Regularity
3. Chain Flexibility
4. Polarity
C. Morphology of Crystalline Polymers
1. Crystal Structure of Polymers
2. Morphology of Polymer Single Crystals Grown from Solution
3. Morphology of Polymers Crystallized from the Melt
VI. Crystallinity and Polymer Properties
VII. Problems
References
Chapter Four
Thermal Transitions in Polymers
I. Introduction
II. The Glass Transition Temperature
A. Molecular Motion and Glass Transition
B. Theories of Glass Transition and Measurement of the Glass Transition Temperature
1. Kinetic Theory
2. Equilibrium Theory
3. Free Volume Theory
C. Factors Affecting Glass Transition Temperature
1. Chain Flexibility
2. Geometric Factors
3. Interchain Attractive Forces
4. Copolymerization
5. Molecular Weight
6. Cross-Linking and Branching
7. Crystallinity
8. Plasticization
III. The Crystalline Melting Point
A. Factors Affecting the Crystalline Melting Point, TM
1. Intermolecular Bonding.
2. Effect of Structure
Copyright 2000 by CRC Press LLC
3. Chain Flexibility
4. Copolymerization
IV. Problems
References
Chapter Five
Polymer Modification
I. Introduction
II. Copolymerization
A. Styrene-Butadiene Copolymers
1. Styrene-Butadiene Rubber (SBR) (Random Copolymer)
2. Styrene-Butadiene Block Polymers
B. Ethylene Copolymers
C. Acrylonitrile-Butadiene-Styrene Copolymers (ABS)
D. Condensation Polymers
1. Acetal Copolymer
2. Epoxies
3. Urea-Formaldehyde (UF) Resins
III. Postpolymerization Reactions
A. Reactions of Polysaccharides
1. Cellulose Derivations
2. Starch and Dextrins
B. Cross-Linking
1. Unsaturated Polyesters
2. Vulcanization
D. Block and Graft Copolymer Formation
1. Block Copolymerization
2. Graft Copolymerization
E. Surface Modification
IV. Functional Polymers
A. Polyurethanes
B. Polymer-Bound Stabilizers
1. Antioxidants
2. Flame Retardants
3. Ultraviolet Stabilizers
C. Polymers in Drug Administration
1. Controlled Drug Release, Degradable Polymers
2. Site-Directed (Targeted) Drug Delivery
V. Problems
References
PART II: POLYMER PREPARATION AND PROCESSING METHODS
Chapter Six
Condensation (Step-Reaction) Polymerization
I. Introduction
II. Mechanism of Condensation Polymerization
III. Kinetics of Condensation Polymerization
IV. Stoichiometry in Linear Systems
V. Molecular Weight Control
VI. Molecular Weight Distribution in Linear Condensation Systems
VII. Molecular Weight Averages
VIII. Ring Formation vs. Chain Polymerization
IX. Three-Dimensional Network Step-Reaction Polymers
Copyright 2000 by CRC Press LLC
X. Prediction of the Gel Point
XI. Morphology of Cross-Linked Polymers
XII. Problems
References
Chapter Seven
Chain-Reaction (Addition) Polymerization
I. Introduction
II. Vinyl Monomers
III. Mechanism of Chain Polymerization
A. Initiation
1. Generation of Free Radicals
B. Propagation
C. Termination
D. Chain Transfer
IV. Steady-State Kinetics of Free-Radical Polymerization
A. Initiation
B. Propagation
C. Termination
V. Autoacceleration (Trommsdorff Effect)
VI. Kinetic Chain Length
VII. Chain-Transfer Reactions
A. Transfer to Undiluted Monomer
B. Transfer to Solvent
VIII. Temperature Dependence of Degree of Polymerization
IX. Ionic and Coordination Chain Polymerization
A. Nonradical Chain Polymerization
B. Cationic Polymerization
1. Mechanism
2. Kinetics
C. Anionic Polymerization
1. Mechanism
2. Kinetics
D. Living Polymers
E. Coordination Polymerization
1. Mechanisms
X. Problems
References
Chapter Eight
Copolymerization
I. Introduction
II. The Copolymer Equation
III. Types of Copolymerization
A. Ideal Copolymerization (r1r2 = 1)
B. Alternating Copolymerization (r1 = r2 = 0)
C. Block Copolymerization (r1 > 1, r2 > 1
IV. Polymer Composition Variation with Feed Conversion
V. Chemistry of Copolymerization
A. Monomer Reactivity
B. Radical Reactivity
C. Steric Effects
D. Alternation-Polar Effects
VI. The Q-e Scheme
Copyright 2000 by CRC Press LLC
VII. Problems
References
Chapter Nine
Polymer Additives and Reinforcements
I. Introduction
II. Plasticizers
III. Fillers and Reinforcements (Composites)
IV. Alloys and Blends
V. Antioxidants and Thermal and UV Stabilizers
A. Polymer Stability
1. Nonchain-Scission Reactions
2. Chain-Scission Reactions
3. Oxidative Degradation
4. Hydrolysis and Chemical Degradation
B. Polymer Stabilizers
VI. Flame Retardants
VII. Colorants
VIII. Antistatic Agents (Antistats).
IX. Problems
References
Chapter Ten
Polymer Reaction Engineering
I. Introduction
II. Polymerization Processes
A. Homogeneous Systems
1. Bulk (Mass) Polymerization
B. Solution Polymerization
C. Heterogeneous Polymerization
1. Suspension Polymerization
2. Emulsion Polymerization
3. Precipitation Polymerization
4. Interfacial and Solution Polycondensations
III. Polymerization Reactors
A. Batch Reactors
B. Tubular (Plug Flow) Reactor
C. Continuous Stirred Tank Reactor (CSTR)
IV. Problems
References
Chapter Eleven
Unit Operations in Polymer Processing
I. Introduction
II. Extrusion
A. The Extruder
B. Extrusion Processes.
III. Injection Molding
A. The Injection Unit
B. The Plasticizing Screw
C. The Heating Cylinder
D. The Clamp Unit
E. Auxiliary Systems
F. The Injection Mold
Copyright 2000 by CRC Press LLC
IV. Blow Molding
A. Process Description
B. Extrusion Blow Molding
C. Injection Blow Molding
V. Rotational Molding
A. Process Description
B. Process Variables
VI. Thermoforming
A. Process Description
1. Vacuum Forming
2. Mechanical Forming
3. Air Blowing Process
B. Process Variables
VII. Compression and Transfer Molding
A. Compression Molding
B. Transfer Molding
VIII. Casting
A. Process Description
B. Casting Processes
1. Casting of Acrylics
2. Casting of Nylon
IX. Problems
References
PART III: PROPERTIES AND APPLICATIONS
Chapter Twelve
Solution Properties of Polymers
I. Introduction
II. Solubility Parameter (Cohesive Energy Density)
III. Conformations of Polymer Chains on Solution
A. End-to-End Dimensions
B. The Freely Jointed Chain
C. Real Polymer Chains
1. Fixed Bond Angle (Freely Rotating)
2. Fixed Bond Angles (Restricted Rotation)
3. Long-Range Interactions
IV. Thermodynamics of Polymer Solutions
A. Ideal Solution
B. Liquid Lattice Theory (Flory-Huggins Theory)
1. Entropy of Mixing
2. Heat and Free Energy of Mixing
C. Dilute Polymer Solutions (Flory–Krigbaum Theory)
D. Osmotic Pressure of Polymer Solutions
V. Solution Viscosity
A. Newton’s Law of Viscosity
B. Parameters for Characterizing Polymer Solution Viscosity
C. Molecular Size and Intrinsic Viscosity
D. Molecular Weight from Intrinsic Viscosity
VI. Problems
References
Copyright 2000 by CRC Press LLC
Chapter Thirteen
Mechanical Properties of Polymers
I. Introduction
II. Mechanical Tests
A. Stress–Strain Experiments
B. Creep Experiments
C. Stress Relaxation Experiments
D. Dynamic Mechanical Experiments
E. Impact Experiments
III. Stress–Strain Behavior of Polymers
A. Elastic Stress–Strain Relations
IV. Deformation of Solid Polymers
V. Compression vs. Tensile Tests
VI. Effects of Structural and Environmental Factors on Mechanical Properties
A. Effect of Molecular Weight
B. Effect of Cross-Linking
C. Effect of Crystallinity
D. Effect of Copolymerization
E. Effect of Plasticizers
F. Effect of Polarity
G. Steric Factors
H. Effect of Temperature
I. Effect of Strain Rate
J. Effect of Pressure
VII. Polymer Fracture Behaviour
A. Brittle Fracture
B. Linear Elastic Fracture Mechanics (LEFM)
VIII. Problems
References
Chapter Fourteen
Polymer Viscoelasticity
I. Introduction
II. Simple Rheological Responses
A. The Ideal Elastic Response
B. Pure Viscous Flow
C. Rubberlike Elastic
III. Viscoelasticity
IV. Mechanical Models for Linear Viscoelastic Response
A. Maxwell Model
1. Creep Experiment
2. Stress Relaxation Experiment
3. Dynamic Experiment
B. The Voight Element
1. Creep Experiment
2. Stress Relaxation Experiment
3. Dynamic Experiment
C. The Four-Parameter Model
V. Material Response Time — The Deborah Number
VI. Relaxation and Retardation Spectra
A. Maxwell-Weichert Model (Relaxation)
B. Voight-Kelvin (Creep) Model
VII. Superposition Principles
A. Boltzmann Superposition Principle
B. Time-Temperature Superposition Principle
Copyright 2000 by CRC Press LLC
IX. Problems
References
Chapter Fifteen
Polymer Properties and Applications
I. Introduction
II. The Structure of the Polymer Industry
A. Polymer Materials Manufacturers
B. Manufacturers of Chemicals, Additives, and Modifiers
C. Compounding/Formulating
D. The Processor
E. The Fabricator
F. The Finisher
III. Raw Materials for the Polymer Industry.
IV. Polymer Properties and Applications ..
A. Polyethylene
B. Polypropylene (PP)
C. Polystyrene
D. Poly(Vinyl Chloride) (PVC)
V. Other Vinyl Polymers .
A. Poly(Vinyl Acetate) PVAC)
B. Poly(Vinyl Alcohol) (PVAL)
VI. Acrylics
A. Poly(Methyl Methacrylate) (PMMA)
B. Polyacrylates
C. Polyacrylonitrile (PAN)—Acrylic Fibers
VII. Engineering Polymers.
A. Acrylonitrile-Butadiene-Styrene (ABS)
B. Polyacetal (Polyoxymethylene — POM
C. Polyamides (Nylons)
D. Polycarbonate (PC)
E. Poly(Phenylene Oxide (PPO)
F. Poly(Phenylene Sulfide) (PPS)
G. Polysulfones
H. Polyimides
I. Engineering Polyesters
J. Fluoropolymers
K. Ionomers
VIII. Elastomers
A. Diene-Based Elastomers
1. Polybutadiene (Butadiene Rubber, BR)
2. Syrene-Butadiene Rubber (SBR)
3. Acrylonitrile-Butadiene Rubber (Nitrile Rubber, NBR)
4. Polyisoprene
5. Polychloroprene (Neoprene)
6. Butyl Rubber
B. Ethylene-Propylene Rubbers
C. Polyurethanes
D. Silicone Elastomers
E. Thermoplastic Elastomers (TPE)
1. Styrene Block Copolymers (Styrenics)
2. Thermoplastic Polyurethane Elastomers (TPUs)
3. Polyolefin Blends
4. Thermoplastic Copolyesters (COPE)
5. Thermoplastic Polyamides
Copyright 2000 by CRC Press LLC
IX. Thermosets
A. Phenolic Resins
B. Amino Resins
C. Epoxy Resins
D. Network Polyester Resins
X. Problems
References
Appendix I Polymer Nomenclature
Appendix II Answers to Selected Problems
Appendix III Conversion Factors
Solutions to Problems
Copyright 2000 by CRC Press LLC
Chapter 1
Introduction
I.
HISTORICAL DEVELOPMENT
Before we go into details of the chemistry of polymers it is appropriate to briefly outline a few landmarks
in the historical development of what we now know as polymers. Polymers have been with us from the
beginning of time; they form the very basis (building blocks) of life. Animals, plants — all classes of
living organisms — are composed of polymers. However, it was not until the middle of the 20th century
that we began to understand the true nature of polymers. This understanding came with the development
of plastics, which are true man-made materials that are the ultimate tribute to man’s creativity and
ingenuity. As we shall see in subsequent discussions, the use of polymeric materials has permeated every
facet of our lives. It is hard to visualize today’s world with all its luxury and comfort without man-made
polymeric materials.
The plastics industry is recognized as having its beginnings in 1868 with the synthesis of cellulose
nitrate. It all started with the shortage of ivory from which billiard balls were made. The manufacturer
of these balls, seeking another production method, sponsored a competition. John Wesley Hyatt (in the
U.S.) mixed pyroxin made from cotton (a natural polymer) and nitric acid with camphor. The result was
cellulose nitrate, which he called celluloid. It is on record, however, that Alexander Parkes, seeking a
better insulating material for the electrical industry, had in fact discovered that camphor was an efficient
plasticizer for cellulose nitrate in 1862. Hyatt, whose independent discovery of celluloid came later, was
the first to take out patents for this discovery.
Cellulose nitrate is derived from cellulose, a natural polymer. The first truly man-made plastic came
41 years later (in 1909) when Dr. Leo Hendrick Baekeland developed phenol–formaldehyde plastics
(phenolics), the source of such diverse materials as electric iron and cookware handles, grinding wheels,
and electrical plugs. Other polymers — cellulose acetate (toothbrushes, combs, cutlery handles, eyeglass
frames); urea–formaldehyde (buttons, electrical accessories); poly(vinyl chloride) (flooring, upholstery,
wire and cable insulation, shower curtains); and nylon (toothbrush bristles, stockings, surgical sutures) —
followed in the 1920s.
Table 1.1 gives a list of some plastics, their year of introduction, and some of their applications. It
is obvious that the pace of development of plastics, which was painfully slow up to the 1920s, picked
up considerable momentum in the 1930s and the 1940s. The first generation of man-made polymers was
the result of empirical activities; the main focus was on chemical composition with virtually no attention
paid to structure. However, during the first half of the 20th century, extensive organic and physical
developments led to the first understanding of the structural concept of polymers — long chains or a
network of covalently bonded molecules. In this regard the classic work of the German chemist Hermann
Staudinger on polyoxymethylene and rubber and of the American chemists W. T. Carothers on nylon
stand out clearly. Staudinger first proposed the theory that polymers were composed of giant molecules,
and he coined the word macromolecule to describe them. Carothers discovered nylon, and his fundamental research (through which nylon was actually discovered) contributed considerably to the elucidation of the nature of polymers. His classification of polymers as condensation or addition polymers
persists today.
Following a better understanding of the nature of polymers, there was a phenomenal growth in the
numbers of polymeric products that achieved commercial success in the period between 1925 and 1950.
In the 1930s, acrylic resins (signs and glazing); polystyrene (toys, packaging and housewares industries);
and melamine resins (dishware, kitchen countertops, paints) were introduced.
The search for materials to aid in the defense effort during World War II resulted in a profound
impetus for research into new plastics. Polyethylene, now one of the most important plastics in the
world, was developed because of the wartime need for better-quality insulating materials for such
applications as radar cable. Thermosetting polyester resins (now used for boatbuilding) were developed
for military use. The terpolymer acrylonitrile-butadiene-styrene (ABS), (telephone handsets, luggage,
0-8493-????-?/97/$0.00+$.50
© 1997 by CRC Press LLC
Copyright 2000 by CRC Press LLC
2
POLYMER SCIENCE AND TECHNOLOGY
Table 1.1 Introduction of Plastics Materials
Date
Material
Typical Use
1868
1909
1919
1926
1927
1927
1929
1935
1936
1936
1938
1938
1938
1938
1939
1939
1942
1942
1943
1943
1945
1947
1948
1949
1954
1956
1957
1957
1959
1962
1962
1964
1964
1964
1964
1965
1965
1970
1971
1973
1974
1975
Cellulose nitrate
Phenol–formaldehyde
Casein
Alkyds
Cellulose acetate
Poly(vinyl chloride)
Urea–formaldehyde
Ethyl cellulose
Polyacrylonitrile
Poly(vinyl acetate)
Cellulose acetate butyrate
Polystyrene
Nylon (polyamide)
Poly(vinyl acetal)
Poly(vinylidene chloride)
Melamine–formaldehyde
Polyester (cross-linkable)
Polyethylene (low density)
Fluoropolymers
Silicone
Cellulose propionate
Epoxies
Acrylonitrile-butadiene-styrene copolymer
Allylic
Polyurethane
Acetal resin
Polypropylene
Polycarbonate
Chlorinated polyether
Phenoxy resin
Polyallomer
Ionomer resins
Polyphenylene oxide
Polyimide
Ethylene–vinyl acetate
Polybutene
Polysulfone
Thermoplastic polyester
Hydroxy acrylates
Polybutylene
Aromatic polyamides
Nitrile barrier resins
Eyeglass frames
Telephone handsets, knobs, handles
Knitting needles
Electrical insulators
Toothbrushes, packaging
Raincoats, flooring
Lighting fixtures, electrical switches
Flashlight cases
Brush backs, displays
Flashbulb lining, adhesives
Irrigation pipe
Kitchenwares, toys
Gears, fibers, films
Safety glass interlayer
Auto seat covers, films, paper, coatings
Tableware
Boat hulls
Squeezable bottles
Industrial gaskets, slip coatings
Rubber goods
Automatic pens and pencils
Tools and jigs
Luggage, radio and television cabinets
Electrical connectors
Foam cushions
Automotive parts
Safety helmets, carpet fiber
Appliance parts
Valves and fittings
Adhesives, coatings
Typewriter cases
Skin packages, moldings
Battery cases, high temperature moldings
Bearings, high temperature films and wire coatings
Heavy gauge flexible sheeting
Films
Electrical/electronic parts
Electrical/electronic parts
Contact lenses
Piping
High-strength tire cord
Containers
safety helmets, etc.) owes its origins to research work emanating from the wartime crash program on
large-scale production of synthetic rubber.
The years following World War II (1950s) witnessed great strides in the growth of established plastics
and the development of new ones. The Nobel-prize-winning development of stereo-specific catalysts by
Professors Karl Ziegler of Germany and Giulio Natta of Italy led to the ability of polymer chemists to
“order” the molecular structure of polymers. As a consequence, a measure of control over polymer
properties now exists; polymers can be tailor-made for specific purposes.
The 1950s also saw the development of two families of plastics — acetal and polycarbonates. Together
with nylon, phenoxy, polyimide, poly(phenylene oxide), and polysulfone they belong to the group of
plastics known as the engineering thermoplastics. They have outstanding impact strength and thermal
and dimensional stability — properties that place them in direct competition with more conventional
materials like metals.
Copyright 2000 by CRC Press LLC
3
INTRODUCTION
The 1960s and 1970s witnessed the introduction of new plastics: thermoplastic polyesters (exterior
automotive parts, bottles); high-barrier nitrile resins; and the so-called high-temperature plastics, including such materials as polyphenylene sulfide, polyether sulfone, etc. The high-temperature plastics were
initially developed to meet the demands of the aerospace and aircraft industries. Today, however, they
have moved into commercial areas that require their ability to operate continuously at high temperatures.
In recent years, as a result of better understanding of polymer structure–property relationships, introduction of new polymerization techniques, and availability of new and low-cost monomers, the concept
of a truly tailor-made polymer has become a reality. Today, it is possible to create polymers from different
elements with almost any quality desired in an end product. Some polymers are similar to existing
conventional materials but with greater economic values, some represent significant improvements over
existing materials, and some can only be described as unique materials with characteristics unlike any
previously known to man. Polymer materials can be produced in the form of solid plastics, fibers,
elastomers, or foams. They may be hard or soft or may be films, coatings, or adhesives. They can be
made porous or nonporous or can melt with heat or set with heat. The possibilities are almost endless
and their applications fascinating. For example, ablation is the word customarily used by the astronomers
and astrophysicists to describe the erosion and disintegration of meteors entering the atmosphere. In this
sense, long-range missiles and space vehicles reentering the atmosphere may be considered man-made
meteors. Although plastic materials are generally thermally unstable, ablation of some organic polymers
occurs at extremely high temperatures. Consequently, selected plastics are used to shield reentry vehicles
from the severe heat generated by air friction and to protect rocket motor parts from hot exhaust gases,
based on the concept known as ablation plastics. Also, there is a “plastic armor” that can stop a bullet,
even shell fragments. (These are known to be compulsory attire for top government and company officials
in politically troubled countries.) In addition, there are flexible plastics films that are used to wrap your
favorite bread, while others are sufficiently rigid and rugged to serve as supporting members in a building.
In the years ahead, polymers will continue to grow. The growth, from all indications, will be not
only from the development of new polymers, but also from the chemical and physical modification of
existing ones. Besides, improved fabrication techniques will result in low-cost products. Today the
challenges of recycling posed by environmental problems have led to further developments involving
alloying and blending of plastics to produce a diversity of usable materials from what have hitherto been
considered wastes.
II.
BASIC CONCEPTS AND DEFINITIONS
The word polymer is derived from classical Greek poly meaning “many” and meres meaning “parts.”
Thus a polymer is a large molecule (macromolecule) built up by the repetition of small chemical units.
To illustrate this, Equation 1.1 shows the formation of the polymer polystyrene.
n CH2
CH
CH2
CH
(1.1)
n
styrene (monomer)
polystyrene (polymer)
(1)
(2)
The styrene molecule (1) contains a double bond. Chemists have devised methods of opening this double
bond so that literally thousands of styrene molecules become linked together. The resulting structure,
enclosed in square brackets, is the polymer polystyrene (2). Styrene itself is referred to as a monomer,
which is defined as any molecule that can be converted to a polymer by combining with other molecules
of the same or different type. The unit in square brackets is called the repeating unit. Notice that the
structure of the repeating unit is not exactly the same as that of the monomer even though both possess
identical atoms occupying similar relative positions. The conversion of the monomer to the polymer
involves a rearrangement of electrons. The residue from the monomer employed in the preparation of a
Copyright 2000 by CRC Press LLC
4
POLYMER SCIENCE AND TECHNOLOGY
polymer is referred to as the structural unit. In the case of polystyrene, the polymer is derived from a
single monomer (styrene) and, consequently, the structural unit of the polystyrene chain is the same as its
repeating unit. Other examples of polymers of this type are polyethylene, polyacrylonitrile, and polypropylene. However, some polymers are derived from the mutual reaction of two or more monomers that
are chemically similar but not identical. For example, poly(hexamethylene adipamide) or nylon 6,6 (5)
is made from the reaction of hexamethylenediamine (3) and adipic acid (4) (Equation 1.2).
H
H2N
(CH2)6
NH2 + HOOC
(CH2)4
COOH
H
N
(CH2)6
H
O
N
C
O
(CH2)4
C
OH
(1.2)
n
hexamethylenediamine
adipic acid
poly(hexamethylene adipamide)
(3)
(4)
(5)
H
|
H
|
The repeating unit in this case consists of two structural units: – N–(CH2)6 – N–, the residue from hexamO
\
O
\
ethylenediamine; and – C–(CH 2) 4– C–, the residue from adipic acid. Other polymers that have repeating
units with more than one structural unit include poly(ethyleneterephthalate) and proteins. As we shall
see later, the constitution of a polymer is usually described in terms of its structural units.
The subscript designation, n, in Equations 1.1 and 1.2 indicates the number of repeating units strung
together in the polymer chain (molecule). This is known as the degree of polymerization (DP). It specifies
the length of the polymer molecule. Polymerization occurs by the sequential reactions of monomers, which
means that a successive series of reactions occurs as the repeating units are linked together. This can proceed
by the reaction of monomers to form a dimer, which in turn reacts with another monomer to form a trimer
and so on. Reaction may also be between dimers, trimers, or any molecular species within the reaction
mixture to form a progressively larger molecule. In either case, a series of linkages is built between the
repeating units, and the resulting polymer molecule is often called a polymer chain, a description which
emphasizes its physical similarity to the links in a chain. Low-molecular-weight polymerization products
such as dimers, trimers, tetramers, etc., are referred to as oligomers. They generally possess undesirable
thermal and mechanical properties. A high degree of polymerization is normally required for a material to
develop useful properties and before it can be appropriately described as a polymer. Polystyrene, with a
degree of polymerization of 7, is a viscous liquid (not of much use), whereas commercial grade polystyrene
is a solid and the DP is typically in excess of 1000. It must be emphasized, however, that no clear
demarcation has been established between the sizes of oligomers and polymers.
The degree of polymerization represents one way of quantifying the molecular length or size of a
polymer. This can also be done by use of the term molecular weight (MW). By definition, MW(Polymer) =
DP × MW(Repeat Unit). To illustrate this let us go back to polystyrene (2). There are eight carbon atoms
and eight hydrogen atoms in the repeating unit. Thus, the molecular weight of the repeating unit is 104
(8 × 12 + 1 × 8). If, as we stated above, we are considering commercial grade polystyrene, we will be
dealing with a DP of 1000. Consequently, the molecular weight of this type of polystyrene is 104,000.
As we shall see later, molecular weight has a profound effect on the properties of a polymer.
Example 1.1: What is the molecular weight of polypropylene (PP), with a degree of polymerization of
3 × 104?
Solution: Structure of the repeating unit for PP
CH2
CH
CH3
Molecular weight of repeat unit = (3 × 12 + 6 × 1) = 42
Molecular weight of polypropylene = 3 × 104 × 42 = 1.26 × 106
Copyright 2000 by CRC Press LLC
(Str. 1)
5
INTRODUCTION
Figure 1.1 Molecular weight distribution curve.
So far, we have been discussing a single polymer molecule. However, a given polymer sample (like
a piece of polystyrene from your kitchenware) is actually composed of millions of polymer molecules.
For almost all synthetic polymers irrespective of the method of polymerization (formation), the length
of a polymer chain is determined by purely random events. Consequently, any given polymeric sample
contains a mixture of molecules having different chain lengths (except for some biological polymers
like proteins, which have a single, well-defined molecular weight [monodisperse]). This means that a
distribution of molecular weight exists for synthetic polymers. A typical molecular weight distribution
curve for a polymer is shown in Figure 1.1.
The existence of a distribution of molecular weights in a polymer sample implies that any experimental
measurement of molecular weight in the given sample gives only an average value. Two types of
molecular weight averages are most commonly considered: the number-average molecular weight represented by Mn, and the weight-average molecular weight Mw . The number-average molecular weight
is derived from measurements that, in effect, count the number of molecules in the given sample. On
the other hand, the weight-average molecular weight is based on methods in which the contribution of
each molecule to the observed effect depends on its size.
In addition to the information on the size of molecules given by the molecular weights Mw and Mn,
their ratio Mw /Mn is an indication of just how broad the differences in the chain lengths of the constituent
polymer molecules in a given sample are. That is, this ratio is a measure of polydispersity, and consequently it is often referred to as the heterogeneity index. In an ideal polymer such as a protein, all the
polymer molecules are of the same size (Mw = Mn or Mw /Mn = 1). This is not true for synthetic polymers –
the numerical value of Mw is always greater than that of Mn. Thus as the ratio Mw /Mn increases, the
molecular weight distribution is broader.
Example 1.2: Nylon 11 has the following structure
H
N
O
(CH2)10
C
(Str. 2)
n
If the number-average degree of polymerization, Xn, for nylon is 100 and Mw = 120,000, what is its
polydispersity?
Solution: We note that Xn and n(DP) define the same quantity for two slightly different entities. The
degree of polymerization for a single molecule is n. But a polymer mass is composed of millions of
molecules, each of which has a certain degree of polymerization. Xn is the average of these. Thus,
Copyright 2000 by CRC Press LLC
6
POLYMER SCIENCE AND TECHNOLOGY
N
∑n M
i
Xn =
r
i =1
N
where N = total number of molecules in the polymer mass
Mr = molecular weight of repeating unit
ni = DP of molecule i.
Now Mn = XnMr = 100 (15 + 14 × 10 + 28)
= 18,300
Polydispersity =
Mw 120, 000
=
= 6.56
Mn
18, 300
III.
CLASSIFICATION OF POLYMERS
Polymers can be classified in many different ways. The most obvious classification is based on the origin
of the polymer, i.e., natural vs. synthetic. Other classifications are based on the polymer structure,
polymerization mechanism, preparative techniques, or thermal behavior.
A. NATURAL VS. SYNTHETIC
Polymers may either be naturally occurring or purely synthetic. All the conversion processes occurring
in our body (e.g., generation of energy from our food intake) are due to the presence of enzymes. Life
itself may cease if there is a deficiency of these enzymes. Enzymes, nucleic acids, and proteins are
polymers of biological origin. Their structures, which are normally very complex, were not understood
until very recently. Starch — a staple food in most cultures — cellulose, and natural rubber, on the other
hand, are examples of polymers of plant origin and have relatively simpler structures than those of
enzymes or proteins. There are a large number of synthetic (man-made) polymers consisting of various
families: fibers, elastomers, plastics, adhesives, etc. Each family itself has subgroups.
B. POLYMER STRUCTURE
1. Linear, Branched or Cross-linked, Ladder vs. Functionality
As we stated earlier, a polymer is formed when a very large number of structural units (repeating units,
monomers) are made to link up by covalent bonds under appropriate conditions. Certainly even if the
conditions are “right” not all simple (small) organic molecules possess the ability to form polymers. In
order to understand the type of molecules that can form a polymer, let us introduce the term functionality.
The functionality of a molecule is simply its interlinking capacity, or the number of sites it has available
for bonding with other molecules under the specific polymerization conditions. A molecule may be
classified as monofunctional, bifunctional, or polyfunctional depending on whether it has one, two, or
greater than two sites available for linking with other molecules. For example, the extra pair of electrons
in the double bond in the styrene molecules endows it with the ability to enter into the formation of two
bonds. Styrene is therefore bifunctional. The presence of two condensable groups in both hexamethylenediamine (–NH2) and adipic acid (–COOH) makes each of these monomers bifunctional. However,
functionality as defined here differs from the conventional terminology of organic chemistry where, for
example, the double bond in styrene represents a single functional group. Besides, even though the
interlinking capacity of a monomer is ordinarily apparent from its structure, functionality as used in
polymerization reactions is specific for a given reaction. A few examples will illustrate this.
A diamine like hexamethylenediamine has a functionality of 2 in amide-forming reactions such as
that shown in Equation 1.2. However, in esterification reactions a diamine has a functionality of zero.
Butadiene has the following structure:
CH2› CH–CH› CH2
1 2 3
4
(6)
Copyright 2000 by CRC Press LLC
(Str. 3)
7
INTRODUCTION
From our discussion about the polymerization of styrene, the presence of two double bonds on the
structure of butadiene would be expected to prescribe a functionality of 4 for this molecule. Butadiene
may indeed be tetrafunctional, but it can also have a functionality of 2 depending on the reaction
conditions (Equation 1.3).
CH2
n CH2
1
CH
CH
CH2
2
3
4
CH
1,2 or
3,4
CH
CH2
n
(7)
1,4
CH2
(1.3)
CH
CH
CH2
n
(8)
Since there is no way of making a distinction between the 1,2 and 3,4 double bonds, the reaction of
either double bond is the same. If either of these double bonds is involved in the polymerization reaction,
the residual or unreacted double bond is on the structure attached to the main chain [i.e., part of the
pendant group (7)]. In 1,4 polymerization, the residual double bond shifts to the 2,3 position along the
main chain. In either case, the residual double bond is inert and is generally incapable of additional
polymerization under the conditions leading to the formation of the polymer. In this case, butadiene has
a functionality of 2. However, under appropriate reaction conditions such as high temperature or crosslinking reactions, the residual unsaturation either on the pendant group or on the backbone can undergo
additional reaction. In that case, butadiene has a total functionality of 4 even though all the reactive sites
may not be activated under the same conditions. Monomers containing functional groups that react under
different conditions are said to possess latent functionality.
Now let us consider the reaction between two monofunctional monomers such as in an esterification
reaction (Equation 1.4).
O
R
COOH + R´
acid
(9)
OH
R
alcohol
(10)
C
O
(1.4)
R´
ester
(11)
You will observe that the reactive groups on the acid and alcohol are used up completely and that the
product ester (11) is incapable of further esterification reaction. But what happens when two bifunctional
molecules react? Let us use esterification once again to illustrate the principle (Equation 1.5).
O
HOOC
R
COOH + HO
bifunctional
(12)
R´ OH
bifunctional
(13)
HOOC
R
C
O
R´
OH
(1.5)
bifunctional
(14)
The ester (14) resulting from this reaction is itself bifunctional, being terminated on either side by
groups that are capable of further reaction. In other words, this process can be repeated almost indefinitely.
The same argument holds for polyfunctional molecules. It is thus obvious that the generation of a polymer
through the repetition of one or a few elementary units requires that the molecule(s) must be at least
bifunctional.
Copyright 2000 by CRC Press LLC
8
POLYMER SCIENCE AND TECHNOLOGY
Figure 1.2 Linear, branched, and cross-linked polymers.
The structural units resulting from the reaction of monomers may in principle be linked together in
any conceivable pattern. Bifunctional structural units can enter into two and only two linkages with other
structural units. This means that the sequence of linkages between bifunctional units is necessarily linear.
The resulting polymer is said to be linear. However, the reaction between polyfunctional molecules
results in structural units that may be linked so as to form nonlinear structures. In some cases the side
growth of each polymer chain may be terminated before the chain has a chance to link up with another
chain. The resulting polymer molecules are said to be branched. In other cases, growing polymer chains
become chemically linked to each other, resulting in a cross-linked system (Figure 1.2).
The formation of a cross-linked polymer is exemplified by the reaction of epoxy polymers, which
have been used traditionally as adhesives and coatings and, more recently, as the most common matrix
in aerospace composite materials. Epoxies exist at ordinary temperatures as low-molecular-weight
viscous liquids or prepolymers. The most widely used prepolymer is diglycidyl ether of bisphenol A
(DGEBA), as shown below (15):
O
CH2
O
CH3
CH
CH2
O
C
O
CH2
CH
CH2
(Str. 4)
CH3
diglycidyl ether of bisphenol A (DGEBA)
(15)
The transformation of this viscous liquid into a hard, cross-linked three-dimensional molecular
network involves the reaction of the prepolymer with reagents such as amines or Lewis acids. This
reaction is referred to as curing. The curing of epoxies with a primary amine such as hexamethylenediamine involves the reaction of the amine with the epoxide. It proceeds essentially in two steps:
1. The attack of an epoxide group by the primary amine
O
H2N
R
NH2 + CH2
1°amine 1°amine
(16)
Copyright 2000 by CRC Press LLC
H
CH
epoxide
(17)
H2N
R
1°amine
N
OH
CH2
2°amine
(18)
CH
(1.6)
9
INTRODUCTION
2. The combination of the resulting secondary amine with a second epoxy group to form a branch
point (19).
CH
H
H2N
R
N
1°amine
OH
CH2
O
CH
+ CH2
2°amine
OH
CH2
CH
R
H2N
epoxide
N
OH
CH2
(1.7)
CH
branch point
(19)
The presence of these branch points ultimately leads to a cross-linked infusible and insoluble polymer
with structures such as (20).
CH
OH
CH
OH
OH
CH2
CH2
N
R
N
CH2
CH
(Str. 5)
CH2
CH
OH
(20)
In this reaction, the stoichiometric ratio requires one epoxy group per amine hydrogen. Consequently,
an amine such as hexamethylenediamine has a functionality of 4. Recall, however, that in the reaction
of hexamethylenediamine with adipic acid, the amine has a functionality of 2. In this reaction DGEBA
is bifunctional since the hydroxyl groups generated in the reaction do not participate in the reaction.
But when the curing of epoxies involves the use of a Lewis acid such as BF3, the functionality of each
epoxy group is 2; that is, the functionality of DGEBA is 4. Thus the curing reactions of epoxies further
illustrate the point made earlier that the functionality of a given molecule is defined for a specific reaction.
By employing different reactants or varying the stoichiometry of reactants, different structures can be
produced and, consequently, the properties of the final polymer can also be varied.
Polystyrene (2), polyethylene (21), polyacrylonitrile (22), poly(methyl methacrylate) (23), and
poly(vinyl chloride) (24) are typical examples of linear polymers.
CH3
CH2
CH2
CH2
n
(21)
CH
CN
(22)
CH2
n
C
C
CH2
Cl
O
n
(24)
O
CH3
(Str. 6)
CH
n
(23)
O\
Substituent groups such as –CH3 , –O– C–CH3, –Cl, and –CN that are attached to the main chain of
skeletal atoms are known as pendant groups. Their structure and chemical nature can confer unique
properties on a polymer. For example, linear and branched polymers are usually soluble in some solvent
at normal temperatures. But the presence of polar pendant groups can considerably reduce room temperature solubility. Since cross-linked polymers are chemically tied together and solubility essentially
Copyright 2000 by CRC Press LLC
10
POLYMER SCIENCE AND TECHNOLOGY
involves the separation of solute molecules by solvent molecules, cross-linked polymers do not dissolve,
but can only be swelled by liquids. The presence of cross-linking confers stability on polymers. Highly
cross-linked polymers are generally rigid and high-melting. Cross-links occur randomly in a cross-linked
polymer. Consequently, it can be broken down into smaller molecules by random chain scission. Ladder
polymers constitute a group of polymers with a regular sequence of cross-links. A ladder polymer, as
the name implies, consists of two parallel linear strands of molecules with a regular sequence of crosslinks. Ladder polymers have only condensed cyclic units in the chain; they are also commonly referred
to as double-chain or double-strand polymers. A typical example is poly(imidazopyrrolone) (27), which
is obtained by the polymerization of aromatic dianhydrides such as pyromellitic dianhydride (25) or
aromatic tetracarboxylic acids with ortho-aromatic tetramines like 1,2,4,5-tetraaminobenzene (26):
O
O
C
C
O
O
C
NH2
H2N
NH2
+
C
O
H2N
O
(25)
(26)
(Str. 7)
O
C
N
N
C
C
N
N
C
O
n
(27)
The molecular structure of ladder polymers is more rigid than that of conventional linear polymers.
Numerous members of this family of polymers display exceptional thermal, mechanical, and electrical
behavior. Their thermal stability is due to the molecular structure, which in essence requires that two
bonds must be broken at a cleavage site in order to disrupt the overall integrity of the molecule; when
only one bond is broken, the second holds the entire molecule together.
Example 1.3: Show the polymer formed by the reaction of the following monomers. Is the resulting
polymer linear or branched/cross-linked?
i.
OCN
(CH2)x
NCO + HO
CH2
CH
(CH2)n
(Str. 8)
CH2OH
OH
ii.
CH2
CH
CN + CH2
H2N
iii.
Copyright 2000 by CRC Press LLC
NH2
CH
H2N
(Str. 9)
CH
CH2
CH
COOH
+
NH2
HOOC
CH2
CH2
(Str. 10)
CH
COOH
11
INTRODUCTION
OH
O
iv.
H2N
C
NH2 + HO
CH2
(Str. 11)
CH2OH
CH2OH
v.
O
CH
CH
C
C
O
+ HO
(CH2)n
OH
CH2
CH
(Str. 12)
O
Solution:
i.
OCN
(CH2)x
NCO + HO
H
C
N
CH2
OH
(Str. 13)
OH
polyfunctional
bifunctional
O
(CH2)n
(CH2)x
H
O
N
C
O
CH2
CH
(CH2)nCH2
O
O
branched/cross-linked
ii.
CH2
CH
+
CH2
(Str. 14)
CH
CN
polyfunctional
CH2
CH
CN
linear
Copyright 2000 by CRC Press LLC
polyfunctional
CH2
CH
12
POLYMER SCIENCE AND TECHNOLOGY
H2N
iii.
NH2
CH
CH2
COOH
CH
+
H2N
HOOC
CH2
CH2
NH2
COOH
polyfunctional
polyfunctional
H
H
N
N
CH
CH2
(Str. 15)
CH
O
CH
C
N
N
C
H
H
O
CH2CH2
CH
C
O
branched/cross-linked
O
iv.
H2N
C
NH2 + HOCH2
CH2OH
(Str. 16)
CH2OH
bifunctional
H
O
N
C
O
polyfunctional
CH2
CH2O
CH2O
branched/cross-linked
The resulting secondary hydrogens in the urea linkages are capable of additional reaction depending on
the stoichiometric proportions of reactants. This means that, in principle, the urea molecule may be
polyfunctional (tetrafunctional).
v.
O
CH
CH
C
C
O
+ HO
O
bifunctional
(CH2)n
OH
(Str. 17)
bifunctional
O
C
CH
CH
C
O
(CH2)n
O
linear
Even though the resulting polymer is linear, it can be cross-linked in a subsequent reaction due to the
unsaturation on the main chain – for example, by using radical initiators.
Copyright 2000 by CRC Press LLC
13
INTRODUCTION
Example 1.4: Explain the following observation. When phthalic acid reacts with glycerol, the reaction
leads first to the formation of fairly soft soluble material, which on further heating yields a hard, insoluble,
infusible material. If the same reaction is carried out with ethylene glycol instead of glycerol, the product
remains soluble and fusible irrespective of the extent of reaction.
Solution:
HO
HOOC
COOH
CH2CHCH2
+
glycerol
OH
O
O
C
C
O
OH
CH2CHCH2
O
O
(Str. 18)
phthalic acid
+
HO
CH2CH2
OH
O
O
C
C
O
CH2CH2
O
ethylene glycol
Phthalic acid and ethylene glycol are both bifunctional. Consequently, only linear polymers are produced
from the reaction between these monomers. On the other hand, the reaction between phthalic acid and
glycerol leads initially to molecules that are either linear, branched, or both. But since glycerol is
trifunctional, cross-linking ultimately takes place between these molecules leading to an insoluble and
infusible material.
2. Amorphous or Crystalline
Structurally, polymers in the solid state may be amorphous or crystalline. When polymers are cooled
from the molten state or concentrated from the solution, molecules are often attracted to each other and
tend to aggregate as closely as possible into a solid with the least possible potential energy. For some
polymers, in the process of forming a solid, individual chains are folded and packed regularly in an
orderly fashion. The resulting solid is a crystalline polymer with a long-range, three-dimensional, ordered
arrangement. However, since the polymer chains are very long, it is impossible for the chains to fit into
a perfect arrangement equivalent to that observed in low-molecular-weight materials. A measure of
imperfection always exists. The degree of crystallinity, i.e., the fraction of the total polymer in the
crystalline regions, may vary from a few percentage points to about 90% depending on the crystallization
conditions. Examples of crystalline polymers include polyethylene (21), polyacrylonitrile (22), poly(ethylene terephthalate) (28), and polytetrafluoroethylene (29).
O
CH2CH2
O
O
O
C
C
(Str. 19)
n
(28)
CF2
(Str. 20)
CF2
n
(29)
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POLYMER SCIENCE AND TECHNOLOGY
In contrast to crystallizable polymers, amorphous polymers possess chains that are incapable of
ordered arrangement. They are characterized in the solid state by a short-range order of repeating units.
These polymers vitrify, forming an amorphous glassy solid in which the molecular chains are arranged
at random and even entangled. Poly(methyl methacrylate) (23) and polycarbonate (30) are typical
examples.
CH3
O
O
C
O
CH3
(Str. 21)
C
n
(30)
From the above discussion, it is obvious that the solid states of crystalline and amorphous polymers
are characterized by a long-range order of molecular chains and a short-range order of repeating units,
respectively. On the other hand, the melting of either polymer marks the onset of disorder. There are,
however, some polymers which deviate from this general scheme in that the structure of the ordered
regions is more or less disturbed. These are known as liquid crystalline polymers. They have phases
characterized by structures intermediate between the ordered crystalline structure and the disordered
fluid state. Solids of liquid crystalline polymers melt to form fluids in which much of the molecular
order is retained within a certain range of temperature. The ordering is sufficient to impart some solidlike properties on the fluid, but the forces of attraction between molecules are not strong enough to
prevent flow. An example of a liquid crystalline polymer is polybenzamide (31).
O
H
C
N
(Str. 22)
n
(31)
Liquid crystalline polymers are important in the fabrication of lightweight, ultra-high-strength, and
temperature-resistant fibers and films such as Dupont’s Kevlar and Monsanto’s X-500. The structural
factors responsible for promoting the above classes of polymers will be discussed when we treat the
structure of polymers.
3. Homopolymer or Copolymer
Polymers may be either homopolymers or copolymers depending on the composition. Polymers composed of only one repeating unit in the polymer molecules are known as homopolymers. However,
chemists have developed techniques to build polymer chains containing more than one repeating unit.
Polymers composed of two different repeating units in the polymer molecule are defined as copolymers.
An example is the copolymer (32) formed when styrene and acrylonitrile are polymerized in the same
reactor. The repeating unit and the structural unit of a polymer are not necessarily the same. As indicated
earlier, some polymers such as nylon 6,6 (5) and poly(ethylene terephthalate) (28) have repeating units
composed of more than one structural unit. Such polymers are still considered homopolymers.
Copyright 2000 by CRC Press LLC
15
INTRODUCTION
n CH2
CH + mCH2
CH
CH2
CH
CH2
CN
(Str. 23)
CH
CN
m
n
(32)
The repeating units on the copolymer chain may be arranged in various degrees of order along the
backbone; it is even possible for one type of backbone to have branches of another type. There are
several types of copolymer systems:
• Random copolymer — The repeating units are arranged randomly on the chain molecule. It we
represent the repeating units by A and B, then the random copolymer might have the structure shown
below:
AABBABABBAAABAABBA
(Str. 24)
• Alternating copolymer — There is an ordered (alternating) arrangement of the two repeating units
along the polymer chain:
ABABABABABAB
(Str. 25)
• Block copolymer — The chain consists of relatively long sequences (blocks) of each repeating unit
chemically bound together:
AAAAA
BBBBBBBB
AAAAAAAAA
BBBB
(Str. 26)
• Graft copolymer — Sequences of one monomer (repeating unit) are “grafted” onto a backbone of the
another monomer type:
B
B
B
B
B
B
AAAAAAAAAAAA
Copyright 2000 by CRC Press LLC
B
B
B
B
B
B
B
B
B
B
AAAAAAAA
(Str. 27)
16
POLYMER SCIENCE AND TECHNOLOGY
4. Fibers, Plastics, or Elastomers
Polymers may also be classified as fibers, plastics, or elastomers. The reason for this is related to how
the atoms in a molecule (large or small) are hooked together. To form bonds, atoms employ valence
electrons. Consequently, the type of bond formed depends on the electronic configuration of the atoms.
Depending on the extent of electron involvement, chemical bonds may be classified as either primary
or secondary.
In primary valence bonding, atoms are tied together to form molecules using their valence electrons. This
generally leads to strong bonds. Essentially there are three types of primary bonds: ionic, metallic, and
covalent. The atoms in a polymer are mostly, although not exclusively, bonded together by covalent bonds.
Secondary bonds on the other hand, do not involve valence electrons. Whereas in the formation of
a molecule atoms use up all their valence bonds, in the formation of a mass, individual molecules attract
each other. The forces of attraction responsible for the cohesive aggregation between individual molecules
are referred to as secondary valence forces. Examples are van der Waals, hydrogen, and dipole bonds.
Since secondary bonds do not involve valence electrons, they are weak. (Even between secondary bonds,
there are differences in the magnitude of the bond strengths: generally hydrogen and dipole bonds are
much stronger than van der Waals bonds.) Since secondary bonds are weaker than primary bonds,
molecules must come together as closely as possible for secondary bonds to have maximum effect.
The ability for close alignment of molecules depends on the structure of the molecules. Those
molecules with regular structure can align themselves very closely for effective utilization of the
secondary intermolecular bonding forces. The result is the formation of a fiber. Fibers are linear polymers
with high symmetry and high intermolecular forces that result usually from the presence of polar groups.
They are characterized by high modulus, high tensile strength, and moderate extensibilities (usually less
than 20%). At the other end of the spectrum, there are some molecules with irregular structure, weak
intermolecular attractive forces, and very flexible polymer chains. These are generally referred to as
elastomers. Chain segments of elastomers can undergo high local mobility, but the gross mobility of
chains is restricted, usually by the introduction of a few cross-links into the structure. In the absence of
applied (tensile) stress, molecules of elastomers usually assume coiled shapes. Consequently, elastomers
exhibit high extensibility (up to 1000%) from which they recover rapidly on the removal of the imposed
stress. Elastomers generally have low initial modulus in tension, but when stretched they stiffen. Plastics
fall between the structural extremes represented by fibers and elastomers. However, in spite of the
possible differences in chemical structure, the demarcation between fibers and plastics may sometimes
be blurred. Polymers such as polypropylene and polyamides can be used as fibers and as plastics by a
proper choice of processing conditions.
C. POLYMERIZATION MECHANISM
Polymers may be classified broadly as condensation, addition, or ring-opening polymers, depending on the
type of polymerization reaction involved in their formation. Condensation polymers are formed from a series
of reactions, often of condensation type, in which any two species (monomers, dimers, trimers, etc.) can
react at any time leading to a larger molecule. In condensation polymerization, the stepwise reaction occurs
between the chemically reactive groups or functional groups on the reacting molecules. In the process, a
small molecule, usually water or ammonia, is eliminated. A typical condensation polymerization reaction is
the formation of a polyester through the reaction of a glycol and a dicarboxylic acid (Equation 1.8). Examples
of condensation polymers include polyamides (e.g., nylon 6,6) (5); polyesters (e.g., poly(ethylene terephthalate) (28); and urea-formaldehyde and phenol–formaldehyde resins.
O
nHO
R
OH + nHOOC
R´
COOH
nH
O
R
O
C
O
R´
C
OH + nH20
(1.8)
n
Addition polymers are produced by reactions in which monomers are added one after another to a
rapidly growing chain. The growing polymer in addition polymerization proceeds via a chain mechanism.
Like all chain reactions, three fundamental steps are involved: initiation, propagation, and termination.
Monomers generally employed in addition polymerization are unsaturated (usually with carbon-carbon
Copyright 2000 by CRC Press LLC
17
INTRODUCTION
double bonds). Examples of addition polymers are polystyrene (2), polyethylene (21), polyacrylonitrile
(22), poly(methyl methacrylate) (23), and poly(vinyl chloride) (24).
As the name suggests, ring-opening polymerization polymers are derived from the cleavage and then
polymerization of cyclic compounds. A broad generalization of ring-opening polymerization is shown
in Equation 1.9.
X
n
(CH2)y
(1.9)
X
(CH2)y
n
The nature of the cyclic structure is such that in the presence of a catalyst it undergoes equilibrium
ring-opening to produce a linear chain of degree of polymerization, n. X is usually a heteroatom such as
oxygen or sulfur; it may also be a group such as lactam or lactone. A number of commercially important
polymers are obtained via ring-opening polymerization. Thus, trioxane (33) can be polymerized to yield
polyoxymethylene (34), the most important member of the family of acetal resins, and caprolactam (35)
undergoes ring-opening to yield nylon 6 (36), an important textile fiber used especially for carpets.
O
HC2
CH2
CH2
(Str. 28)
O
n
O
O
CH2
(33)
(34)
O
H
O
C
N
(CH2)5
N
(35)
H
(CH2)5
(Str. 29)
C
n
(36)
We will discuss the various polymerization mechanisms in greater detail in Chapter 2. The original
classification of polymers as either condensation or addition polymers as proposed by Carothers does not
permit a complete differentiation between the two classes or polymers, particularly in view of the new
polymerization processes that have been developed in recent years. Consequently, this classification has
been replaced by the terms step-reaction (condensation) and chain-reaction (addition) polymerization.
These terms focus more on the manner in which the monomers are linked together during polymerization.
D. THERMAL BEHAVIOR
For engineering purposes, the most useful classification of polymers is based on their thermal (thermomechanical) response. Under this scheme, polymers are classified as thermoplastics or thermosets. As
the name suggests, thermoplastic polymers soften and flow under the action of heat and pressure. Upon
cooling, the polymer hardens and assumes the shape of the mold (container). Thermoplastics, when
compounded with appropriate ingredients, can usually withstand several of these heating and cooling
cycles without suffering any structural breakdown. This behavior is similar to that of candle wax.
Examples of thermoplastic polymers are polyethylene, polystyrene, and nylon.
Copyright 2000 by CRC Press LLC
18
POLYMER SCIENCE AND TECHNOLOGY
Figure 1.3 Idealized modulus–temperature curves for thermoplastics and thermosets.
A thermoset is a polymer that, when heated, undergoes a chemical change to produce a cross-linked,
solid polymer. Thermosets usually exist initially as liquids called prepolymers; they can be shaped into
desired forms by the application of heat and pressure, but are incapable of undergoing repeated cycles
of softening and hardening. Examples of thermosetting polymers include urea–formaldehyde, phenol–formaldehyde, and epoxies.
The basic structural difference between thermoplastics and thermosets is that thermoplastic polymers
are composed mainly of linear and branched molecules, whereas thermosets are made up of cross-linked
systems. Recall from our previous discussion that linear and branched polymers consist of molecules
that are not chemically tied together. It is therefore possible for individual chains to slide past one
another. For cross-linked systems, however, chains are linked chemically; consequently, chains will not
flow freely even under the application of heat and pressure.
The differences in the thermal behavior of thermoplastics and thermosets are best illustrated by
considering the change in modulus with temperature for both polymers (Figure 1.3). At low temperatures,
a thermoplastic polymer (both crystalline and amorphous) exists as a hard and rigid glass. As the
temperature is increased, it changes from a glass to a rubbery elastomer to a viscous melt that is capable
of flowing — hence this phase is also known as the flow region. (The transitions between the different
phases or regions of thermal behavior are characterized by drops in the magnitude of the modulus —
usually two to three orders. As we shall see later, differences exist between amorphous and crystalline
thermoplastics in the details and nature of these transitions). For the thermosetting polymer, on the other
hand, the modulus remains high in the rubbery region, while the flow region disappears.
E. PREPARATIVE TECHNIQUE
Polymers can be classified according to the techniques used during the polymerization of the monomer.
In bulk polymerization, only the monomer (and possibly catalyst and initiator, but no solvent) is fed into
the reactor. The monomer undergoes polymerization, at the end of which a (nearly) solid mass is removed
as the polymer product. As we shall see later, bulk polymerization is employed widely in the manufacture
of condensation polymers, where reactions are only mildly exothermic and viscosity is mostly low thus
enhancing ready mixing, heat transfer, and bubble elimination. Solution polymerization involves polymerization of a monomer in a solvent in which both the monomer (reactant) and polymer (product) are
soluble. Suspension polymerization refers to polymerization in an aqueous medium with the monomer
as the dispersed phase. Consequently, the polymer resulting from such a system forms a solid dispersed
phase. Emulsion polymerization is similar to suspension polymerization but the initiator is located in
Copyright 2000 by CRC Press LLC
19
INTRODUCTION
the aqueous phase (continuous phase) in contrast to the monomer (dispersed phase) in suspension
polymerization. Besides, in emulsion polymerization the resulting polymer particles are considerably
smaller (about ten times smaller) than those in suspension polymerization.
F. END USE
Finally, polymers may be classified according to the end use of the polymer. In this case, the polymer
is associated with a specific industry (end use): diene polymers (rubber industry); olefin polymer (sheet,
film, and fiber industries); and acrylics (coating and decorative materials).
IV.
1.1.
PROBLEMS
Show the structural formulae of the repeating units that would be obtained in the polymerization of
the following monomers. Give the names of the polymers.
CH2
CH2
CH
COOH
CH3
O
C
C
O
(Str. 30)
CH3
(Str. 31)
CH3
(Str. 32)
O
CH
CH2
1.2.
O
C
CH2
CH
CH3
(Str. 33)
CH2
CH
CN
(Str. 34)
Show the repeating units that would be obtained from the reaction of the following monomer(s).
O
a.
H2N
(CH2)5 NH2 and Cl
b. HOOC
C
O
(CH2)5
C
(CH2)10
COOH and HO
Cl
(Str. 35)
OH
(Str. 36)
O
c. HO CH2
d.
CH2OH and O
C
C
CH3
NCO and HO
NCO
Copyright 2000 by CRC Press LLC
CH2
CH2
CH2
OH
O
(Str. 37)
(Str. 38)
20
1.3.
POLYMER SCIENCE AND TECHNOLOGY
Complete the following table.
Monomer
Repeat Unit
Polymer
a.
Poly(ethyl acrylate)
CH3
b. CH2
C
CH3
c.
CH2
C
CH3
d.
Poly(vinylidene chloride)
O
e.
C
(CH2)5
N
CH2
f.
H2C
C
O
O
H2C
CH2
CH2
g.
Poly(dimethylsiloxane)
h.
Copyright 2000 by CRC Press LLC
H
O
CH2
CH2
CH2
CH2
21
INTRODUCTION
1.4.
Complete the table by indicating whether the monomer(s) will form a polymer and, if so, whether the
polymer formed will be linear or branched/cross-linked.
Polymer
Yes
Monomer A
Monomer B
O
a. R
HO
R´
No
OH
C
OH
OH
b. HOOC
c. HO
R
R
COOH
CH2
CH
CH2
R´
R´
OH
CH
HO
OH
N
C
HOOC
R´
O
d.
e. HO
(CH2)5 COOH
f. H2N
R
NH2
COOH
NH2
g.
CH
O
CH
C
C
CH
CH2
CH
CH2
O
O
h. H N
2
i. CH
2
R
NH2
OCN
R´
NCO
CHCOOH
H2N CH2
j. CH O
2
k. CH2
O
Copyright 2000 by CRC Press LLC
CH2
CH2NH2
CH2
O
O
Linear
Branched/
Cross-linked
22
1.5.
POLYMER SCIENCE AND TECHNOLOGY
What is the molecular weight of the following polymers if the degree of polymerization is 1000?
H
a.
N
b.
O
c.
CH2
O
(CH2)5
(Str. 39)
C
CH2CH2CH2
O
O
O
C
C
(Str. 40)
(Str. 41)
CH
CH3
CH3
d.
O
O
C
O
(Str. 42)
C
CH3
1.6.
Draw the structural formulae of the repeating units of the following polymers.
a.
b.
c.
d.
Poly(ethylene succinate)
Poly(ethylene sebacate)
Poly(hexamethylene phthalate)
Poly(tetramethylene oxalate)
1.7.
A polyester is formed by a condensation reaction between maleic anhydride and diethylene glycol.
Styrene is then added and polymerized. Describe the chemical composition and molecular architecture
of the resulting polymer. What would be the effect if maleic anhydride were replaced with adipic acid?
1.8.
Natural rubber is a polymer of isoprene CH2
C
C
(Str. 43)
CH2
CH3 H
a. Show what structures can form as it polymerizes.
b. What feature of the polymer chain permits vulcanization?
1.9.
An industrialist wants to set up a phenol–formaldehyde adhesive plant. He has approached you with
the following phenolic compounds.
OH
R
OH
R
OH
R
(Str. 44)
R
R
R
(a)
(b)
R
(c)
Which of the compounds (a, b, or c) would you choose for reaction with formaldehyde? Explain your
choice.
1.10. The following structure represents the general formula for some aliphatic amines.
H2N [ (CH2)2
NH]n
(CH2)2
NH2
(Str. 45)
What is the functionality of the corresponding amine in its reaction with diglycidyl ether of bisphenol A
(DGEBA) if n = 1, 2, 3, 4?
Copyright 2000 by CRC Press LLC
23
INTRODUCTION
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
Frados, J., The Story of the Plastics Industry, Society of the Plastics Industry, New York, 1977.
Billmeyer, F.W., Jr., Textbook of Polymer Science, 3rd ed., Interscience, New York, 1984.
Fried, J.R., Plast. Eng., 38(6), 49, 1982.
Fried, J.R., Plast. Eng., 38(7), 27, 1982.
Fried, J.R., Plast. Eng., 38(11), 27, 1982.
Fried, J.R., Plast. Eng., 38(12), 21, 1982.
Fried, J.R., Plast. Eng., 39(3), 67, 1983.
Kaufman, H.S., 1969/70 Modern Plastics Encyclopedia, McGraw-Hill, New York, 1969, 29.
Williams, D.J., Polymer Science and Engineering, Prentice-Hall, Englewood Cliffs, NJ, 1971.
Kaufman H.S. and Falcetta, J.J., eds., Introduction to Polymer Science and Technology, John Wiley & Sons, New
York, 1977.
Rudin, A., The Elements of Polymer Science and Engineering, Academic Press, New York, 1982.
Flory, P.J., Principles of Polymer Chemistry, Cornell University Press, Ithaca, NY, 1952.
Carothers, W.H., Chem. Rev., 8(3), 353, 1931.
Wendorff, J.H., Finkelmann, H., and Ringsdorf, H., J. Polym. Sci. Polym. Symp., 63, 245, 1978.
Braunsteiner, E.E., J. Polym. Sci. Macromol. Rev., 9, 83, 1974.
McGrath, J.E., Makromol. Chem. Macromol. Symp., 42/43, 69, 1991.
Copyright 2000 by CRC Press LLC
Chapter 2
Polymerization Mechanisms
I.
INTRODUCTION
As discussed in Chapter 1, under a scheme proposed by Carothers, polymers are classified as addition
or condensation polymers depending on the type of polymerization reaction involved in their synthesis.
This classification scheme, however, does not permit a complete differentiation between the two classes
of polymers. A more complete but still oversimplified scheme that is still based on the different
polymerization processes places polymers into three classes: condensation, addition, and ring-opening
polymers. This scheme reflects the structures of the starting monomers. Probably the most general
classification scheme is based on the polymerization mechanism involved in polymer synthesis. Under
this scheme, polymerization processes are classified as step-reaction (condensation) or chain-reaction
(addition) polymerization. In this chapter, we will discuss the different types of polymers based on the
different polymerization mechanisms.
II.
CHAIN-REACTION POLYMERIZATION
Chain-reaction polymerization, an important industrial method of polymer preparation, involves the
addition of unsaturated molecules to a rapidly growing chain. The most common unsaturated compounds
that undergo chain-reaction polymerization are olefins, as exemplified by the following reaction of a
generalized vinyl monomer.
nCH2
CH
CH2
R
(2.1)
CH
R
n
The growing polymer in chain-reaction polymerization is a free radical, and polymerization proceeds
via chain mechanism. Chain-reaction polymerization is induced by the addition of free-radical-forming
reagents or by ionic initiators. Like all chain reactions, it involves three fundamental steps: initiation,
propagation, and termination. In addition, a fourth step called chain transfer may be involved.
A. INITIATION
Initiation involves the acquisition of an active site by the monomer. This may occur spontaneously by the
absorption of heat, light (ultraviolet), or high-energy irradiation. But most frequently, initiation of freeradical polymerization is brought about by the addition of small quantities of compounds called initiators.
Typical initiators include peroxides, azo compounds, Lewis acids, and organometallic reagents. However,
while initiators trigger initiation of the chain and exert an accelerating influence on polymerization rate,
they are not exactly catalysts since they are changed chemically in the course of polymerization. An
initiator is usually a weak organic compound that can be decomposed thermally or by irradiation to
produce free radicals, which are molecules containing atoms with unpaired electrons. A variety of
compounds decompose when heated to form free radicals. Dialkyl peroxides (ROOR), diacylperoxides
(RCO–O–O–CO–R), hydroperoxides (ROOH), and azo compounds (RN› NR) are typical organic compounds that can be decomposed thermally to produce free radicals. Benzoyl peroxide, azobisisobutyronitrile, and di-t-butylperoxide are commonly used free-radical initiators, as illustrated in Equations 2.2–2.4.
O
C
O
O
O
O
C
C
2
benzoyl peroxide
0-8493-????-?/97/$0.00+$.50
© 1997 by CRC Press LLC
Copyright 2000 by CRC Press LLC
O•
2
•
+ 2CO2
(2.2)
26
POLYMER SCIENCE AND TECHNOLOGY
CH3
C
CH3
CH3
N
N
CN
C
CH3
CH3
C • + N2
2CH3
CN
(2.3)
CN
azobisisobutyronitrile (AIBN)
CH3
CH3
C
CH3
CH3
O
O
C
CH3
CH3
2CH3
CH3
C
O•
(2.4)
CH3
di-t-butylperoxide
The thermal decomposition of benzoyl peroxide, which takes place between 60 and 90°C, involves the
homolytic cleavage of the O–O bond to yield benzoyl free radicals that may react to yield phenyl radicals
and carbon dioxide. An example of photochemically induced free-radical formation is the decomposition
of azo-bisisobutyronitrile by short-wavelength visible light or near-ultraviolet radiation at temperatures
as low as 0°C, where no thermal initiation occurs.
In free-radical polymerization carried out in aqueous medium, the decomposition of peroxide or persulfate is greatly accelerated by the presence of a reducing system. This method of free-radical initiation
is referred to as redox initiation. The initiation resulting from the thermal decomposition of organic
compounds discussed above is appropriate only for polymerizations carried out at room temperature or
higher. The enhanced rate of free-radical formation in redox reactions permits polymerization at relatively
lower temperatures. Typical redox reactions for emulsion polymerization are shown in Equations 2.5–2.7.
S2O82− + HSO3− → SO24− + SO 4− ∑+ HSO3 ∑
(2.5)
S2O82− + S2O3− → SO24− + SO −4 ∑+ S2O3− ∑
(2.6)
HSO3− + Fe3+ → HSO3 ∑+ Fe 2 +
(2.7)
Persulfate ion initiator (e.g., from K2S2O8) reacts with a reducing agent such as a bisulfite ion (e.g., from
NaHSO3) to produce radicals for redox initiation (Equations 2.5 and 2.6). Ferric ion may also be used
as a source of radicals (Equation 2.7). Other redox reactions involve the use of alkyl hydroxides and a
reducing agent such as ferrous ion (Equation 2.8).
CH3
C
CH3
CH3
OOH + Fe2+
C
O • + Fe3+
(2.8)
CH3
As indicated earlier, free-radical polymerization of some monomers can be initiated by heating or
exposing the monomers to light or high-energy irradiation such as X-rays, γ-rays, and α-rays. High-energy
irradiation of monomers can be carried out either in bulk or in solution. It is certainly not as selective
as photolytic initiation.
When choosing an initiator for free-radical polymerization, the important parameters that must be
considered are the temperature range to be used for the polymerization and the reactivity of the radicals
formed. The presence of certain promoters and accelerators and the nature of the monomer often affect
the rate of decomposition of initiators. For example, the decomposition of benzoyl peroxide may be
accelerated at room temperature by employing ternary or quaternary amines. Free-radical initiation
Copyright 2000 by CRC Press LLC
27
POLYMERIZATION MECHANISMS
processes do not require stringent exclusion of atmospheric moisture, but can be inhibited by substances
such as oxygen. Free radicals are inactivated by reaction with oxygen to form peroxides or hydroperoxides. For monomers such as styrene and methylmethacrylate that are susceptible to such inhibition,
initiation reactions are carried out in an oxygen-free atmosphere such as nitrogen. It must be emphasized
also that organic peroxides, when subjected to shock or high temperature, can detonate. Therefore these
compounds must be handled with caution.
The initiation of polymerization occurs in two successive steps. The first step involves the formation
of radicals according to the processes discussed above. This may be represented broadly as:
I - I → 2I ⋅
(2.9)
The second step is the addition of the initiator radical to a vinyl monomer molecule:
H
I • + CH2
C
H
I
CH2
(2.10)
C•
R
R
Initiator fragments have been shown by end-group analysis to become part of the growing chain. In
commercial practice, 60 to 100% of all the free radicals generated do initiate polymerization.
B. PROPAGATION
During propagation, the initiated monomer described above adds other monomers — usually thousands
of monomer molecules — in rapid succession. This involves the addition of a free radical to the double
bond of a monomer, with regeneration of another radical. The active center is thus continuously relocated
at the end of the growing polymer chain (Equation 2.11).
H
I
CH2
H
C • + CH2
CHR
I
CH2
CH
R
C•
CH2
R
(2.11)
R
Propagation continues until the growing chain radical is deactivated by chain termination or transfer as
discussed below.
The substituted carbon atom is regarded as the head and the unsubstituted carbon atom the tail of
the vinyl monomer. There are, therefore, three possible ways for the propagation step to occur: headto-tail (Equation 2.11), head-to-head (Equation 2.12), and tail-to-tail (Equation 2.13). A random distribution of these species along the molecular chain might be expected. It is found, however, that head-totail linkages in which the substituents occur on alternate carbon atoms predominate; only occasional
interruptions of this arrangement by head-to-head and tail-to-tail linkages occur. In addition, exclusive
head-to-head or tail-to-tail arrangements of monomers in the chain are now known.
H
I
CH2
C • + CH2
CHR
I
CH2
H
H
C
C
R
R
R
H
I
CH2
C • + CH2
R
Copyright 2000 by CRC Press LLC
H
CHR
I
C
R
CH2 •
(2.12)
H
CH2
CH2
C•
R
(2.13)
28
POLYMER SCIENCE AND TECHNOLOGY
C. TERMINATION
In termination, the growth activity of a polymer chain radical is destroyed by reaction with another free
radical in the system to produce polymer molecule(s). Termination can occur by the reaction of the
polymer radical with initiator radicals (Equation 2.14). This type of termination process is unproductive
and can be controlled by maintaining a low rate for initiation.
H
I
CH2
C • + I´
I
CH2
R
H
H
C
C
R
R
(2.14)
I´
The termination reactions that are more important in polymer production are combination (or coupling)
and disproportionation. In termination by combination, two growing polymer chains react with the mutual
destruction of growth activity (Equation 2.15), while in disproportionation a labile atom (usually hydrogen) is transferred from one polymer radical to another (Equation 2.16).
H
I
CH2
H
C• + •C
R
H
I
CH2
I
I
CH2
R
H
C• + •C
R
CH2
CH2
I
I
CH2
R
H
H
C
C
R
R
CH2
H
H
C
H + C
R
R
CH
(2.15)
I
I
(2.16)
Coupling reactions produce a single polymer, while disproportionation results in two polymers from the
two reacting polymer chain radicals. The predominant termination reaction depends on the nature of the
reacting monomer and the temperature. Since disproportionation requires energy for breaking of chemical
bonds, it should become more pronounced at high reaction temperatures; combination of growing
polymer radicals predominates at low temperatures.
D. CHAIN TRANSFER
Ideally, free-radical polymerization involves three basic steps: initiation, propagation, and termination,
as discussed above. However, a fourth step, called chain transfer, is usually involved. In chain-transfer
reactions, a growing polymer chain is deactivated or terminated by transferring its growth activity to a
previously inactive species, as illustrated in Equation 2.17.
H
I
CH2
C • + TA
R
H
I
CH2
C
T + A•
(2.17)
R
The species, TA, could be a monomer, polymer, solvent molecule, or other molecules deliberately or
inadvertently introduced into the reaction mixture. Depending on its reactivity, the new radical, A·, may
or may not initiate the growth of another polymer chain. If the reactivity of A· is comparable to that of
the propagating chain radical, then a new chain may be initiated. If its reactivity toward a monomer is
less than that of the propagating radical, then the overall reaction rate is retarded. If A· is unreactive
toward the monomer, the entire reaction could be inhibited. Transfer reactions do not result in the creation
or destruction of radicals; at any instant, the overall number of growing radicals remains unchanged.
However, the occurrence of transfer reactions results in the reduction of the average polymer chain
length, and in the case of transfer to a polymer it may result in branching.
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29
POLYMERIZATION MECHANISMS
E. DIENE POLYMERIZATION
Conjugated dienes such as butadiene (1), chloroprene (2), and isoprene (3) constitute a second group of
unsaturated compounds that can undergo polymerization through their double bonds.
Cl
CH2
CH
CH
(1)
CH2
CH2
C
CH3
CH
(2)
CH2
CH2
C
CH
CH2
(Str. 1)
(3)
These structures contain double bonds in the 1,2 and 3,4 positions, each of which may participate
independently in polymerization giving rise to 1,2 and 3,4 units. A further possibility is that both bonds
are involved in polymerization through conjugate reactions, resulting in 1,4 units. These structures are
shown in Equation 2.18.
(2.18)
Diene polymerization thus gives rise to polymers that contain various isomeric units. With symmetrical
dienes such as butadiene, the 1,2 and 3,4 units are identical. The 1,4 unit may occur in the cis or trans
configuration. A diene polymer contains more than one of these structural units. The relative abundance
of each unit in the polymer molecule depends on the nature of the initiator, experimental conditions,
and the structure of the diene. The proportion of each type of structure incorporated into the polymer
chain influences both thermal and physical properties. For example, butadiene can be polymerized by
free-radical addition at low temperature to produce a polymer that consists almost entirely of trans-1,4
units and only about 20% 1,2 units. As the temperature is increased, the relative proportion of cis-1,4
units increases while the proportion of 1,2 structure remains fairly constant. Anionic diene polymerization
with lithium or organolithium initiators like n-butyllithium in nonpolar solvents such as pentane or
hexane yields polymers with high cis-1,4 content. When higher alkali metal initiators or more polar
solvents are used, the relative amount of cis-1,4 units decreases. Stereoregularity can also be controlled
by the use of coordination catalysts like Ziegler–Natta catalysts. Heterogeneous Alfin catalysts — which
are combinations of alkenyl sodium compounds, alkali metal halides, and an alkoxide — give highmolecular-weight polymers with high content of trans-1,4 units.
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30
POLYMER SCIENCE AND TECHNOLOGY
As noted above, all chain-reaction polymerizations involve essentially the same number of steps. The
main distinguishing feature between chain-reaction polymerizations, however, is by the initiation mechanism, which may be a free-radical, ionic (cationic or anionic), or coordination. The time between
initiation and termination of a given chain is typically from a few tenths of a second to a few seconds.
During this time thousands or tens of thousands of monomers add to the growing chain.
The structural unit in addition polymers is chemically identical to the monomer employed in the
polymerization reaction, as exemplified by the following reaction of a generalized vinyl monomer:
H
R
H
R
n C
C
C
C
H
X
H
X
monomer
(2.18A)
n
polymer
Here R and X are monofunctional groups, R may be a hydrogen atom (H), or an alkyl group (e.g.,
–CH3), while X may be any group (e.g., –Cl, –CN). The structural unit is evidently structurally identical
to the starting monomer. Monomers generally employed in addition polymerizations are unsaturated
(usually with double bonds). Because of the identical nature of the chemical formulas of monomers and
the polymers derived from them, addition polymers generally take their names from the starting
monomer — ethylene → polyethlene, propylene → polypropylene, etc. (Table 2.1). The backbone of
addition polymer chains is usually composed of carbon atoms.
Table 2.1
Some Representation Addition Polymers
H
H
H
H
nC
C
C
C
H R
monomer
R
Monomer
H R
polymer
n
Polymer
H
Ethylene
Polyethylene
CH3
Propylene
Polypropylene
Cl
Vinyl chloride
Poly(vinyl chloride)
CN
Acrylonitrile
Polyacrylonitrile
Styrene
Polystyrene
Vinyl acetate
Poly(vinyl acetate)
Methyl acrylate
Poly(methyl acrylate)
O
C
O
CH3
C
O
CH3
Copyright 2000 by CRC Press LLC
O
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POLYMERIZATION MECHANISMS
Table 2.1 (continued)
R1
R2
R3
H
H
CH3
Some Representation Addition Polymers
R1
R3
R1
R3
nC
C
C
C
R2
R4
R2
R4
R4
C
O
n
Monomer
Polymer
Methyl methacrylate
Poly(methyl methacrylate)
Vinylidene chloride
Vinylidene fluoride
Tetrafluoro-ethylene
Poly(vinylidene chloride)
Poly(vinylidene fluoride)
Polytetrafluoroethylene
O
CH3
H
H
F
H
H
F
Cl
F
F
Cl
F
F
Example 2.1: Explain the following observations.
a. α-Methylstyrene polymerizes much less readily than styrene.
b. Chain-transfer reactions reduce the average chain length of the polymer.
Solutions:
a. The reactivity of a vinyl monomer depends on the nature of the substituents on the monomer
double bond. Substituents may either enhance the monomer reactivity by activating the double
bond, depress the reactivity of the resulting radical by resonance stabilization, or provide steric
hindrance at the reaction site.
Monomer
Radical
CH3
CH2
C
CH3
CH2
C•
α - methylstyrene
(Str. 2)
H
CH2
C
H
CH2
C•
styrene
The reactivities of α-methylstyrene and styrene radicals are essentially the same due to their
similar resonance stabilization. However, the activation of the double bond by the phenyl group
is compensated somewhat by the presence of the electron-donating methyl group in α-methylstyrene. The methyl group also provides steric hindrance at the reactive site. Consequently, α-methylstyrene is less reactive than styrene.
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POLYMER SCIENCE AND TECHNOLOGY
b. Chain transfer may be to a solvent, initiator or monomer. During chain-transfer reactions, for each
radical chain initiated, the number of polymer molecules increases except in the case of transfer
to a polymer. In other words, the average number of monomer molecules consumed by each chain
radical (DP) or the average polymer chain length decreases with chain-transfer reactions.
III.
IONIC AND COORDINATION POLYMERIZATIONS
As noted earlier, chain-reaction polymerization may be classified as free-radical, cationic, anionic, or
coordination polymerization depending on the nature of the reactive center. The growing polymer
molecule is associated with counterions in ionic (cationic and anionic) polymerization or with a coordination complex in coordination polymerization. Ionic polymerizations involve chain carriers or reactive
centers that are organic ions or charged organic groups. In anionic polymerization, the growing chain
end carries a negative charge or carbanions, while cationic polymerization involves a growing chain end
with a positive charge or carbonium (carbenium) ion. Coordination polymerization is thought to involve
the formation of a coordination compound between the catalyst, monomer, and growing chain.
The mechanisms of ionic and coordination polymerizations are more complex and are not as clearly
understood as those of free radical polymerization. Here, we will briefly highlight the essential features
of these mechanisms, and more details will be given in Chapter 7. Initiation of ionic polymerization
usually involves the transfer of an ion or an electron to or from the monomer. Many monomers can
polymerize by more than one mechanism, but the most appropriate polymerization mechanism for each
monomer is related to the polarity of the monomers and the Lewis acid–base strength of the ion formed.
A. CATIONIC POLYMERIZATION
Monomers with electron-donating groups like isobutylene form stable positive charges and are readily
converted to polymers by cationic catalysts. Any strong Lewis acid like boron trifluoride (BF3) or
Friedel–Crafts catalysts such as AlCl3 can readily initiate cationic polymerization in the presence of a
cocatalyst like water, which serves as a Lewis base or source of protons. During initiation, a proton adds
to the monomer to form a carbonium ion, which forms an association with the counterion. This is
illustrated for isobutylene and boron trifluoride in Equation 2.19:
CH3
BF3• H2O + CH2
C
CH3
CH3
CH3
C⊕
BF3OH
(2.19)
CH3
Propagation involves the consecutive additions of monomer molecules to the carbonium ion at the
growing chain end. Termination in cationic polymerization usually involves rearrangement to produce
a polymer with an unsaturated terminal unit and the original complex or chain transfer to a monomer
and possibly to the polymer or solvent molecule. Unlike free-radical polymerization, termination by
combination of two cationic polymer growing chains does not occur.
Cationic polymerizations are usually conducted in solutions and frequently at temperatures as low
as –80 to –100°C. Polymerization rates at these low temperature conditions are usually fast. The cation
and the counterion in cationic polymerization remain in close proximity. If the intimate association
between the ion pair is too strong, however, monomer insertion during propagation will be prevented.
Therefore the choice of solvent in cationic polymerization has to be made carefully; a linear increase
in polymer chain length and an exponential increase in the reaction rate usually occur as the dielectric
strength of the solvent increases.
B. ANIONIC POLYMERIZATION
Monomers that are suitable for anionic polymerization generally contain electron-withdrawing substituent groups. Typical monomers include styrene, acrylonitrile, butadiene, methacrylates, acrylates, ethylene oxide, and lactones. The initiator in anionic polymerization may be any compound providing a
strong nucleophile, including Grignard reagents and other organometallic compounds. Initiation involves
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33
POLYMERIZATION MECHANISMS
the addition of the initiator to the double bond of the monomer, as illustrated for styrene and butyllithium
in Equation 2.20.
n
C4H9- Li+ + CH2
CH2
n
C4H9
CH2
CH- Li+
(2.20)
The reaction produces a carbanion at the head end to which is associated the positively charged lithium
counterion. Propagation occurs by the successive insertion of monomer molecules by anionic attack of
the carbanion. No chain transfer or branching occurs in anionic polymerization, particularly if reactions
are carried out at low temperatures. Termination of the growth activity of the polymer chain takes place
either by the deliberate or accidental introduction into the system of oxygen, carbon dioxide, methanol,
water, or other molecules that are capable of reacting with the active chain ends. We note that in anionic
polymerization as well as free-radical polymerization, the initiator or part of it becomes part of the resulting
polymer molecule, attached to the nongrowing chain end. This contrasts with cationic polymerization
where the catalyst is necessary for initiation and propagation, but is regenerated at the termination step.
In some systems, termination can be avoided if the starting reagents are pure and the polymerization
reactor is purged of all oxygen and traces of water. This produces polymer molecules that can remain
active even after all the monomer molecules are consumed. When fresh monomer is added, polymerization resumes. Such polymeric molecules are referred to as “living polymers” because of the absence
of termination. Since the chain ends grow at the same rate, the molecular weight of living polymers is
determined simply by the ratio of monomer concentration to that of the initiator (Equation 2.21).
Degree of Polymerization (DP) =
[monomer]
[initiator]
(2.21)
Polymers produced by living polymerization are characterized by very narrow molecular weight distribution (Poisson distribution). The polydispersity D is given by Equation 2.22
D = Mw M n = 1 + 1 DP
(2.22)
—
—
where M w and M nare the weight-average molecular weight and number-average molecular weight,
respectively.
As discussed in Chapter 7, the absence of termination in living polymerization permits the synthesis
of unusual and unique block polymers — star- and comb-shaped polymers. Living polymerization can
also be employed to introduce a variety of desired functional groups at one or both ends of polymeric
chains both in homo- and block polymers. In particular, living polymerization techniques provide the
synthetic polymer chemist with a vital and versatile tool to control the architecture of a polymer;
complicated macromolecules can be synthesized to meet the rigid specification imposed by a scientific
or technological demand.
C. COORDINATION POLYMERIZATION
As we shall see in Chapter 4, monomers with side groups asymmetrically disposed with respect to the
double bond are capable of producing polymers in which the side groups have a specific stereochemical
or spatial arrangement (isotactic or syndiotactic). In both cationic and anionic polymerizations, the
association of initiating ion and counterion permits a preferential placement of asymmetric substituted
monomers, the extent of which depends on the polymerization conditions. Unbranched and stereospecific
polymers are also produced by the use of Ziegler–Natta catalysts. These are complex catalyst systems
derived from a transition metal compound from groups IVB to VIIIB of the periodic table and an
organometallic compound usually from a group IA or IIIA metal. A typical catalyst complex is that
formed by trialkyl aluminum and titanium trichloride as shown below:
Cl
R
Ti
Cl
Copyright 2000 by CRC Press LLC
Rl
Al
Cl
R
(Str. 3)
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POLYMER SCIENCE AND TECHNOLOGY
Monoolefins such as propylene and dienes such as butadiene and isoprene can be polymerized using
Ziegler–Natta coordination catalysts. The catalysts function by forming transient π-complexes between
the monomers and the transition metal species. The initiating species is a metal–alkyl complex and
propagation involves the consecutive insertion of monomer molecules into a polarized titanium–carbon
bond. Coordination polymerizations may be terminated by introducing poisons such as water, hydrogen,
aromatic alcohols, or metals like zinc into the reacting system.
IV.
STEP-GROWTH POLYMERIZATION
Step-growth polymerization involves a series of reactions in which any two species (monomers, dimers,
trimers, etc.) can react at any time, leading to a larger molecule. Most step-growth polymerizations, as
we shall see presently, involve a classical condensation reaction such as esterification, ester interchange,
or amidization. In step-growth polymerization, the stepwise reaction occurs between pairs of chemically
reactive or functional groups on the reacting molecules. In most cases, step-growth polymerization is
accompanied by the elimination of a small molecule such as water as a by-product. A typical step-growth
polymerization of the condensation type is the formation of a polyester through the reaction of a glycol
and a dicarboxylic acid, as shown in Equation 2.23
nHO
OH + nHOOC
R
O
nH
R
O
C
R´
COOH
n
OH + nH2O
O
R´
C
(2.23)
where R and R′ are the unreactive part of the molecules.
Step-growth polymerizations generally involve either one or more types of monomers. In either case,
each monomer has at least two reactive (functional) groups. In cases where only one type of monomer
is involved, which is known as A-B step-growth polymerization, the functional groups on the monomer
are different and capable of intramolecular reactions. An example is the formation of an aliphatic
polyester by the self-condensation of ω-hydroxycaproic acid (Equation 2.24).
O
nHO
(CH2)5
C
ω-hydroxycproic acid
O
OH
(CH2)5
C
+ 2nH2O
O
(2.24)
n
polycaprolactone
Here, each molecule contains two different functional groups: a hydroxyl group (–OH) and a carboxylic
O
||
acid group (–COOH). These react to form a series of ester linkages (– C–O–) shown in the shaded box.
In those cases where more than one type of molecule is involved, the functional groups on each type of
monomer are the same, but capable of intermolecular reaction with the other type of monomer. This is
known as the A–A/B–B step-growth polymerization and is exemplified by the preparation of poly(ethylene terephthalate) and nylon 6,6 (Equations 2.25 and 2.26).
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POLYMERIZATION MECHANISMS
nHO
O
O
C
C
OH + nHO
terephthalic acid
CH2CH2
OH
ethylene glycol
O
C
CH2 CH2
+ 2nHO
O
(2.25)
n
poly(ethylene terephthalate)
(2.26)
In Equation 2.25, for example, poly(ethylene terephthalate) is formed from the condensation of a
dicarboxylic acid and a diol.
Step-growth polymerizations can be divided into two main categories: polycondensation, in which a
small molecule is eliminated at each step, as discussed above; and polyaddition, in which, as the name
suggests, monomers react without the elimination of a small molecule. These are shown in Equations 2.27
and 2.28, respectively, where R and R′ are the nonreactive portions of the molecules.
A − R − A + B − R ′ − B → A − R − R ′ − B + AB
(2.27)
polycondensation
A − R − A + B − R ′ − B → A − R − AB − R ′ − B
(2.28)
polyaddition
An example of polyaddition-type step-growth polymerization is the preparation of polyurethane by the
ionic addition of diol (1,4 butanediol) to a diisocyanate (1,6 hexane diisocyanate) (Equation 2.29).
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36
POLYMER SCIENCE AND TECHNOLOGY
nHO
(CH2)4
OH + nO
1,4-butanediol
C
N
N
(CH2)6
C
O
1,6-hexane diisocyanate
basic
catalyst
O
O
(CH2)4
O
H
C
N
(CH2)6
H
O
N
C
(2.29)
n
polyurethane
Another example of polyaddition-type step-growth polymerization is the preparation of polyurea from
the reaction of diisocyanate and diamine, as shown in Equation 2.30.
nH2N
(CH2)6
NH2 + nO
C
hexamethylenediamine
N
(CH2)6
N
C
O
hexamethylene diisocyanate
basic
catalyst
H
N
(CH2)6
H
O
H
N
C
N
(CH2)6
H
O
N
C
(2.30)
n
polyurea
The characteristic linkage (group) in each of the above classes of polymers shown in the boxes has been
summarized in Table 2.2.
In contrast to addition polymers, the structural unit in step-growth polymers is not identical chemically
to the structure of the starting monomer(s). Consequently, step-growth polymers derive their names from
the reactive type (characteristic interunit linkage) involved in the polymerization process. In the reaction
between the glycol and dicarboxylic acid, for instance, the resulting polymer is a polyester, in consonance
with the general name of reactions between hydroxyl groups (–OH) and carboxylic acid groups (–COOH)
(Table 2.2).
By extension of this argument, the chemical structures of condensation polymers are not readily
derived from the names of the polymers. Furthermore, the backbone of condensation polymers is
heterogeneous, being generally composed of carbon plus other atoms, usually nitrogen and oxygen
(Table 2.3) and sometimes sulfur and silicon. As we shall see later, this has serious implications for the
resultant polymer properties. The main distinguishing features between addition and condensation polymerizations are summarized in Table 2.3.
Example 2.2: Unsaturated polyester resins, which are used as the matrix component of glass–fiber
composites, may be obtained by the copolymerization of maleic anhydride and diethylene glycol. The
low-molecular-weight product is soluble in styrene. Describe, with the aid of equations, the possible
structures of the prepolymer and that of the polymer resulting from benzoyl peroxide-initiated polymerization of a solution of the prepolymer in styrene.
Copyright 2000 by CRC Press LLC
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POLYMERIZATION MECHANISMS
TABLE 2.2 Some Functional Groups and Their Characteristic
Interunit Linkage in Polymers
Reactants
Functional Group
Characteristic
Interunit Linkage
Polymer Type
O
OH +
COOH
NH2 +
C
O
O
H
C
N
O
H
O
C
N
H
O
H
N
C
N
COOH
OH +
NCO
NH2 +
NCO
O
COOH +
COOH
OH
OH
+
CH
C
Polyester
Polyamide
Polyurethane
Polyurea
O
O
C
Polyanhydride
O
Polyether
O
Polyether
CH
O
O
HO
C
O
OH
O
C
O
Polycarbonate
TABLE 2.3 Distinguishing Features of Chain and Step Polymerization Mechanisms
Chain Polymerization
Only growth reaction adds repeating unit one at a time of the chain.
Monomer concentration decreases steadily throughout reaction.
High polymer is formed at once; polymer molecular weight changes
little throughout reaction.
Reaction mixture contains only monomer, high polymer, and about
10–5 part of growing chains.
Step Polymerization
Any two molecular species present can react.
Monomer disappears early in reaction: at DP 10, less
than 1% monomer remains.
Polymer molecular weight rises steadily throughout
reaction.
At any stage all molecular species are present in a
calculable distribution.
From Billmeyer, F. W., Jr., Textbook of Polymer Science, 3rd ed., Interscience, New York, 1984. With permission.
Copyright 2000 by CRC Press LLC
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POLYMER SCIENCE AND TECHNOLOGY
Solution:
(Str. 4)
Maleic anhydride is unsaturated. Its reaction with diethylene glycol leads to a prepolymer with residual
double bonds on the main chain. These participate in cross-linking reactions with styrene during initiation
with benzoyl peroxide. The result is a network polymer.
A. TYPICAL STEP-GROWTH POLYMERIZATIONS
1. Polyesters
Polyesters form a large class of commercially important polymers. A typical polyester is poly(ethylene
terephthalate) (PETP), the largest volume synthetic fiber. It is also used as film (mylar) and in bottle
applications. We have already discussed in the preceding section one of the routes for the preparation
of PETP. The traditional route for the production of commercial PETP is through two successive ester
interchange reactions, as shown below:
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39
POLYMERIZATION MECHANISMS
xCH3O
O
O
C
C
OCH3 + 2x HO
dimethyl terephthalate
CH2CH2
OH
ethylene glycol
catalyst
150 - 200°C
nHO
CH2CH2
O
O
O
C
C
H + 2xCH3OH
OCH2CH2CH2O
(Str. 5)
x
catalyst
260 - 300°C
O
O
C
C
O
CH2CH2
O
+ nx HO
CH2CH2
OH •
nx
poly(ethylene terephthalate)
In the first step, a 1:2 molar ratio of dimethyl terephthalate to ethylene glycol is heated at temperatures
near 200°C in the presence of a catalyst such as calcium acetate. During this stage, methanol is evolved
and an oligomeric product (x = 1 to 4) is obtained. The second step involves a temperature increase to
about 300°C. This results in the formation of high polymer with the evolution of ethylene glycol.
Poly(ethylene terephthalate) is a linear polyester obtained from the reaction of difunctional monomers.
Branched or network polyesters are obtained if at least one of the reagents is tri- or multifunctional. This
can be achieved either by the use of polyols such as glycerol in the case of saturated polyesters (glyptal)
or by the use of unsaturated dicarboxylic acids such as maleic anhydride in the case of unsaturated
polyester. In the preparation of glyptal, glycerol and phthalic anhydride react to form a viscous liquid
initially, which on further reaction hardens as a result of network formation (Equation 2.31).
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40
POLYMER SCIENCE AND TECHNOLOGY
(2.31)
Glyptal is used mainly as an adhesive. Glyptal modified with natural or synthetic oils is known as an
alkyd resin, which is a special polyester of great importance in the coatings industry. A typical alkyd
resin comprises the following reagents:
(2.32)
The fatty acid RCOOH may be derived from vegetable drying oils (e.g., soybean, linseed oils) or from
nondrying oils (e.g., coconut oil).
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41
POLYMERIZATION MECHANISMS
2. Polycarbonates
O
||
Polycarbonates are a special class of polyesters derived from carbonic acid (HO–C–OH) and have the
following general structure:
O
R
O
C
(Str. 6)
O
n
Polycarbonates are the second largest by volume engineering thermoplastics next to polyamides. Their
preparation involves the linking together of aromatic dihydroxy compounds, usually 2,2-bis(4-hydroxyphenyl) propane or bisphenol A, by reacting them with a derivative of carbonic acid such as phosgene
(Equation 2.33) or diphenyl carbonate (Equation 2.34).
CH3
O
OH + nCl
C
n HO
C
Cl
(2.33)
CH3
bisphenol A
phosgene
CH3
O
C
O
O
+ 2nHCl
C
n
CH3
polycarbonate
CH3
O
OH + PhO
C
n HO
C
OPH
(2.34)
CH3
bisphenol A
diphenyl carbonate
CH3
O
C
O
O
CH3
+ 2nPhOH
C
n
polycarbonate
The reaction of bisphenol A with phosgene involves bubbling the phosgene into a solution of bisphenol
A in pyridine at 20 to 35°C and isolation of the resulting polymer by precipitation in water or methanol.
In the reaction of Bisphenol A with diphenyl carbonate, a prepolymer is formed initially by heating the
mixture at 180 to 220°C in vacuum. Then the temperature is raised slowly to 280 to 300°C at reduced
pressure to ensure the removal of the final traces of phenol.
3. Polyamides
Polyamides, or nylons, as they are commonly called, are characterized by the presence of amide linkages
(–CONH–) on the polymer main chain. Theoretically, a large number of polyamides can be synthesized
based on four main synthetic routes: (1) condensation reaction between a dicarboxylic acid and a diamine,
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POLYMER SCIENCE AND TECHNOLOGY
(2) reaction between a diacid chloride and a diamine, (3) dehydration–condensation reactions of amino
acids, and (4) ring-opening polymerization of lactams. Chemically, nylons may be divided into two
categories: those based on synthetic routes (1) and (2); and those based on routes (3) and (4). The
commercial use of nylons is centered around two products: nylon 6,6 from the first category, and nylon
6 from the second. We now expatiate our earlier discussion of the preparation of nylon 6,6, while the
preparation of nylon 6 will be deferred to a subsequent section.
As with other polyamides, the classical route for the synthesis of nylon 6,6 is the direct reaction
between a dicarboxylic acid (adipic acid) and a diamine (hexamethylenediamine). In practice, however, to
achieve an exact stoichiometric equivalence between the functional groups, a 1:1 salt of the two reactants
is prepared initially and subsequently heated at a high temperature to form the polyamide. For nylon
6,6, an intermediate hexamethylene diammonium adipate salt is formed. A slurry of 60 to 80% of the
recrystallized salt is heated rapidly. The steam that is released is purged by air. Temperature is then raised
to 220°C and finally to 270 to 280°C when the monomer conversion is about 80 to 90% while maintaining
the steam pressure generated during polymerization at 200 to 250 psi. The pressure is subsequently reduced
to atmospheric pressure, and heating is continued until completion of polymerization:
O
HO
C
O
(CH2)4
C
OH + H2N
adipic acid
(CH2)6
NH2
hexamethylenediamine
heat
O
O
C
O
(CH2)4
C
⊕
O
H3N
(CH2)6
⊕
NH3
(Str. 7)
hexamethylene diammonium adipate salt
heat
O
C
(CH2)4
O
H
C
N
H
(CH2)6
N
+ 2H2O
nylon 6,6
Since the polymerization reaction occurs above the melting points of both reactants and the polymer,
the polymerization process is known as melt polymerization.
Other polyamides of commercial importance are nylons that are higher analogs of the more common
types: nylons 11; 12; 6,10; and 6,12. The numerals in the trivial names refer to the number of carbon
atoms in the monomer(s). In designating A–A/B–B nylons, the first number refers to the number of
carbon atoms in the diamine while the second number refers to the total number of carbon atoms in the
acid. For example, nylon 6,10 is poly(hexamethylene sebacamide) (Equation 2.35).
Copyright 2000 by CRC Press LLC
43
POLYMERIZATION MECHANISMS
O
nH2N
NH2 + nHO
(CH2)6
C
N
(CH2)6
(CH2)8
C
OH
sebacic acid
hexamethylenediamine
H
O
H
O
N
C
O
(CH2)8
C
+ 2nH2O
n
poly(hexamethylene sebacamide)
(nylon 6,10)
(2.35)
In the 1960s aromatic polyamides were developed to improve the flammability and heat resistance
of nylons. Poly(m-phenyleneisophthalamide), or Nomex, is a highly heat resistant nylon obtained from
the solution or interfacial polymerization of a metasubstituted diacid chloride and a diamine
(Equation 2.36).
H2N
NH2
Cl
O
O
C
C
Cl
+
m-phenylenediamine
HCl
isophthaloyl chloride
H
H
O
O
N
N
C
C
(2.36)
n
poly(m-phenyleneisophthalamide)
(Nomex)
The corresponding linear aromatic polyamide is Kevlar aramid which decomposes only above 500°C
(Equation 2.37).
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POLYMER SCIENCE AND TECHNOLOGY
O
O
C
C
HCl
H2N
NH2
+
Cl
p-phenylenediamine
Cl
terephthaloyl chloride
H
H
O
O
N
N
C
C
(2.37)
n
poly(p-phenyleneterephthalamide)
(Kevlar)
The high thermooxidative stability of Kevlar is due to the absence of aliphatic units in its main chain.
The material is highly crystalline and forms a fiber whose strength and modulus are higher than that of
steel on an equal weight basis.
4. Polyimides
Polyimides are condensation polymers obtained from the reaction of dianhydrides with diamines. Polyimides are synthesized generally from aromatic dianhydrides and aliphatic diamines or, in the case of
aromatic polyimides, from the reaction of aromatic dianhydrides with aromatic diamines. Aromatic
polyimides are formed by a general two-stage process. The first step involves the condensation of
aromatic dianhydrides and aromatic diamines in a suitable solvent, such as dimethylacetamide, to form
a soluble precursor or poly(amic acid). This is followed by the dehydration of the intermediate poly(amic
acid) at elevated temperature:
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45
POLYMERIZATION MECHANISMS
O
O
C
C
n O
O
C
C
O
O
pyromellitic dianhydride
+
n H2N
m-phenyldiamine
30 - 40°C
H
O
O
H
N
C
C
N
HOOC
poly(amic acid)
NH2
COOH
n
(2.38)
150 - 250°C
O
O
C
C
N
+
N
C
C
O
O
2nH2O
n
poly(m-phenylpyromellitimide)
(4)
The cured or fully imidized polyimide, unlike the poly(amic acid), is insoluble and infusible with high
thermooxidative stability and good electrical-insulation properties. Thermoplastic polyimides that can
be melt processed at high temperatures or cast in solution are now also available. Through an appropriate
choice of the aromatic diamine, phenyl or alkyl pendant groups or main-chain aromatic polyether linkages
can be introduced into the polymer. The resulting polyimides are soluble in relatively nonpolar solvents.
For polyimides to be useful polymers, they must be processable, which means that they have to be
meltable. Melt processability of polyimides can be improved by combining the basic imide structure
with more flexible aromatic groups. This can be achieved by the use of diamines that can introduce
flexible linkages like aromatic ethers and amides into the backbone. Polyamide-imides (5) are obtained
by condensing trimellitic anhydrides and aromatic diamines, while polyetherimides (6) are produced by
nitro displacement reaction involving bisphenol A, 4,4′-methylenedianiline, and 3-nitrophthalic anhydride.
Copyright 2000 by CRC Press LLC
46
POLYMER SCIENCE AND TECHNOLOGY
O
C
H
N
C
C
O
O
O
(Str. 8)
N
n
(5)
O
O
C
C
N
CH3
C
O
N
C
O
C
CH3
O
O
n
(6)
5. Polybenzimidazoles and Polybenzoxazoles
Aromatic substituents at the chains of vinyl polymers influence the behavior of these materials. Aromatic
units as part of the main chain exert a profound influence on virtually all important properties of the
resulting polymer. Aromatic polyamides are formed by the repetitive reaction of aromatic amino group
and carboxyl group in the molar ratio of 1:1. In aromatic polyamides as well as aromatic polyesters, the
chain-stiffening aromatic rings are separated from each other by three consecutive single bonds:
(Str. 9)
The two tetrahedral angles associated with these bonds permit some degree of chain flexibility, which
limits the mechanical and thermal properties of the resulting polymers. One way of reducing flexibility
and enhancing these properties is to reduce the number of consecutive single bonds between two aromatic
units to two, one, or even zero. In polyethers, polysulfides, and polysulfones, as we shall see shortly,
the number of consecutive single bonds has been reduced to two, and these are separated by only one
tetrahedral angle:
O
Copyright 2000 by CRC Press LLC
O
S
S
polyether
polysulfide
polysulfone
(Str. 10)
47
POLYMERIZATION MECHANISMS
Polyimides, polybenzimidazole, and polybenzoxazoles are polymers where the number of these bonds
has been reduced to one. Ladder polymers typify cases where there are no consecutive single bonds
between aromatic moities in the main chain.
In aromatic polyimides (4), two of the three consecutive single bonds between aromatic groups in
polyamides are eliminated by the formation of a new ring. This is achieved by employing a 2:1 molar
ratio of aromatic carboxyl and amino groups. When the molar ratio of carboxyl groups (e.g., terephthalic
acid) to amino groups (e.g., 3,3′ diaminobenzidine) is 1:2, polybenzimidazoles (7) are formed; whereas
where the molar ratio of carboxyl, amino, and hydroxyl groups is 1:1:1, polybenzoxazoles (8) are formed.
HOOC
COOH + H2N
NH2
H2N
NH2
terephthalic acid
3,3´ diaminobenzidine
(2.39)
H
H
N
N
C
C
N
N
n
polybenzimidazole
(7)
COOH +
HOOC
OH
HO
NH3Cl
ClH3N
terephthalic acid
4,6-diamino-1,3-benzenediol dihydrochloride
O
O
C
N
N
polybenzoxazole
(8)
Copyright 2000 by CRC Press LLC
(2.40)
C
n
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POLYMER SCIENCE AND TECHNOLOGY
6. Aromatic Ladder Polymers
As evident from the preceding discussion, the next logical step to increase the rigidity of linear macromolecules is the elimination of single bonds in the main chain so that it is composed of only condensed
cyclic units. The resulting polymer has the following generalized structure (9)
(Str. 11)
(9)
where the individual cyclic units may be either aromatic or cycloaliphatic, homocyclic or heterocyclic.
Polymers of this type are known as ladder polymers. Polybenzimidazoles discussed previously are typical
aromatic ladder polymers. Longer segments of ladder are present in polyimidazopyrrolones (10) prepared
by polymerization of aromatic dianhydrides or aromatic tetracarboxylic acids with ortho-aromatic tetramines according to the following scheme:
O
O
C
C
O
C
O
O
NH2
H2N
NH2
+
O
C
H2N
(2.41)
O
O
C
C
N
C
N
C
N
N
n
(10)
Polyquinoxalines (11) are another group of ladder polymers. They differ from polyimidazopyrrolones
by the presence of a fused six-membered cyclic diimide structure. A possible synthetic route for
polyquinoxalines involves the reaction of 1,4,5,8-naphthalene tetracarboxylic acid with aromatic tetraamines in polyphosphoric acid (PPA) at temperatures up to 220°C.
Copyright 2000 by CRC Press LLC
49
POLYMERIZATION MECHANISMS
HOOC
COOH
H2N
NH2
H2N
NH2
+
HOOC
COOH
PPA
(2.42)
-H2O
O
O
C
C
N
N
C
C
N
N
n
(11)
Ladder polymers are also referred to as “double-chain” or “double-strand” polymers because, unlike
other polymers, the backbone consists of two chains. Cleavage reactions in single-chain polymers cause
a reduction in molecular weight that ultimately results in a deterioration in the properties of the polymer.
For this to happen in a ladder polymer, two bonds will have to be broken in the same chain residue,
which is a very unlikely occurrence. Therefore, ladder polymers usually have exceptional thermal,
mechanical, and electrical properties.
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50
POLYMER SCIENCE AND TECHNOLOGY
Example 2.3: Polyimides have been prepared from aromatic anhydrides and aliphatic diamines by melt
fusion of salt from the diamine and tetracid (dianhydride). Aliphatic polypyromellitimide derived from
straight-chain aliphatic diamines containing more than nine carbon atoms gave thin, flexible films,
whereas those from shorter chain aliphatic diamines allowed the preparation of only thick, brittle
moldings. Explain this observation.
Solution: The general structure of aliphatic polypyromellitimide is represented as follows:
O
O
C
C
N
N
C
C
O
O
(Str. 12)
R
n
where R is an alkylene group. Polyimides are usually rigid polymers. Some flexibility can be introduced
into the polymer structure by incorporating flexible groups in the backbone of the polymer such as
flexible diamines. The longer the alkylene group the more flexible the diamine. It is apparent from the
observation that straight-chain alkylenes with nine or more carbon atoms introduced sufficient flexibility
into the resulting polymer that films made from this polymer were flexible. On the other hand, when R
was less than nine carbon atoms, the resulting polymers were still too rigid and, consequently, brittle.
7. Formaldehyde Resins
Formaldehyde is employed in the production of aminoplasts and phenoplasts, which are two different
but related classes of thermoset polymers. Aminoplasts are products of the condensation reaction between
either urea (urea–formaldehyde or UF resins) or melamine (melamine–formaldehyde or MF resins) with
formaldehyde. Phenoplasts or phenolic (phenol–formaldehyde or PF) resins are prepared from the
condensation products of phenol or resorcinol and formaldehyde.
a. Urea–Formaldehyde Resins
Urea–formaldehyde resin synthesis consists basically of two steps. In the first step, urea reacts with
aqueous formaldehyde under slightly alkaline conditions to produce methylol derivatives of urea (12, 13).
O
H2N
C
NH2 + HO
CH2
urea
OH
formaldehyde (aqueous)
O
H2N
C
OH
CH2
NH
(monomethylol urea)
(12)
HO
CH2
OH
NH
CH2
O
HO
CH2
NH
C
(13)
Copyright 2000 by CRC Press LLC
OH (dimethylol urea)
(2.43)
51
POLYMERIZATION MECHANISMS
In the second step, condensation reactions between the methylol groups occur under acidic conditions,
leading ultimately to the formation of a network structure (14):
O
N
O
C
CH2OH + HOCH2
NH
NH
O
N
N
O
C
NH
CH2
O
O
N
C
C
CH2
NH
C
N
(2.44)
O
NH
CH2
NH
C
N
(14)
b. Melamine–Formaldehyde Resins
Production of melamine–formaldehyde polymers involves reactions essentially similar to those of UF
resins, that is, initial production of methylol derivatives of melamine, which on subsequent condensation,
ultimately form methylene bridges between melamine groups in a rigid network structure (15).
(2.45)
c. Phenol–Formaldehyde Resins
Phenolic resins are prepared by either base-catalyzed (resoles) or acid-catalyzed (novolacs) addition of
formaldehyde to phenol. In the preparation of resoles, phenol and excess formaldehyde react to produce a
mixture of methylol phenols. These condense on heating to yield soluble, low-molecular-weight prepolymers
or resoles (16). On heating of resoles at elevated temperature under basic, neutral, or slightly acidic conditions,
a high-molecular-weight network structure or phenolic rings linked by methylene bridges (17) is produced.
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52
POLYMER SCIENCE AND TECHNOLOGY
(2.46)
Novolacs are low-molecular-weight, fusible but insoluble prepolymers (18) prepared by reaction of
formaldehyde with molar excess of phenol. Novolacs, unlike resoles, do not contain residual methylol
groups. A high-molecular-weight network polymer similar to that of resoles is formed by heating novolac
with additional formaldehyde, paraformaldehyde, or hexamethylenetetramine.
OH
OH
OH
CH2
CH2
CH2
CH2
CH2
(Str. 13)
OH
OH
(18)
We now discuss a number of polymerization reactions that are of the step-reaction type but which
may not necessarily involve condensation reactions.
8. Polyethers
As we said earlier, the introduction of aromatic units into the main chain results in polymers with better
thermal stability than their aliphatic analogs. One such polymer is poly(phenylene oxide), PPO, which
has many attractive properties, including high-impact strength, resistance to attack by mineral and organic
acids, and low water absorption. It is used, usually blended with high-impact polystyrene (HIPS), to
ease processability in the manufacture of machined parts and business machine enclosures.
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53
POLYMERIZATION MECHANISMS
Poly(phenylene oxide) (19) is obtained from free-radical, step-growth, oxidative coupling of 2,6dimethylphenol (2,6-xylenol). This involves passing oxygen into a reaction mixture containing 2,6xylenol, cuprous chloride, and pyridine.
CH3
n
OH
CH3
+
n
2
O2
+ nH2O
O
CH3
(2.47)
CH3
n
(19)
9. Polysulfides
Aromatic polythioethers or polysulfides are closely related to polyethers in structure and properties. A
typical aromatic polysulfide is poly(phenylene sulfide) (PPS) (20), which is used as electrical insulators
and structural parts in the building of engines and vehicles. Poly(phenylene sulfide) is prepared by the
condensation reaction between p-dichlorobenzene and sodium sulfide:
n Cl
S
Cl + nNa2S
(2.48)
+ 2nNaCl
n
(20)
10. Polysulfones
Another family of linear aromatic polymers is the polysulfones. They are tough, high-temperature-resistant
engineering thermoplastics. Polysulfones may be synthesized by the nucleophilic substitution of alkali
salts of biphenates with activated aromatic dihalides. A typical example is the preparation of bisphenol
A polysulfone (21) from the reaction of disodium salt of bisphenol A with dichlorodiphenyl sulfone:
CH3
nNaO
C
O
ONa
+
nCl
S
CH3
Cl
O
disodium salt of bisphenol A
4,4´-dichlorodiphenyl sulfone
(2.49)
CH3
O
C
O
O
CH3
+ 2nNaCl
S
O
n
(21)
The polymerization reaction involves the initial preparation of the disodium salt of bisphenol A by the
addition of aqueous NaOH to bisphenol A in dimethyl sulfoxide (DMSO). A solution of dichlorodiphenyl
sulfone is added, and polymerization is carried out at 160°C.
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54
POLYMER SCIENCE AND TECHNOLOGY
V. RING-OPENING POLYMERIZATION
While ring-opening polymerization shares certain features with condensation and addition polymerization mechanisms, it differs from each of them in at least one important respect. In the first place, in
contrast to condensation polymerization, no small molecule is split off in ring-opening polymerization.
Second, unlike olefin polymerization, the driving force for ring-opening polymerization is not derived
from the loss of unsaturation.
A significant number of polymers has been produced from the ring-opening of cyclic organic compounds, including epoxides such as ethylene and propylene oxides and epichlorohydrin and other cyclic
ethers like trioxane and tetrahydrofuran. Other important systems include cyclic esters (lactones), cyclic
amides (lactams), cycloolefins, and siloxane. Ring-opening polymerization involves essentially an initial
ring-opening of the cyclic monomer followed by polyaddition. The resulting polymers are normally
linear. Their structural units usually have the same composition as the monomer. Major applications of
polymers obtained from ring-opening polymerization are in coatings, fibers, elastomers, adhesives, and
thermoplastics- and thermoset-based composite systems.
Ring-opening polymerizations may be represented broadly by Equation 2.50
X
(CH2)y
(2.50)
X
n
(CH2)y
where X may be a heteroatom such as O, S, or a group like NH, –O–CO–, –NH–CO, or –C› C–. Not
all cyclic compounds can undergo ring-opening polymerization. It is therefore understood from
Equation 2.50 that the cyclic structure is capable of ring-opening polymerization to produce a linear
chain of degree of polymerization, n. The nature of X is such that it provides a mechanism for a catalyst
or initiator to form the initiating coordination intermediate with the cyclic ring. Table 2.4 shows a number
of commercially important polymers produced by ring-opening polymerization together with the associated polymerization initiation mechanism.
A. POLY(PROPYLENE OXIDE)
The polymerization of propylene oxide represents an important example of industrial ring-opening
polymerization. It involves an attack of the least sterically hindered carbon by the hydroxyl anion to
produce the alkoxide (Equation 2.51). This produces essentially linear polymer molecules.
O
KOH + CH2
HO
CH2
CH
CH
CH3
O
K
∆
(2.51)
Propagation
then H+
CH2
CH
O
n
CH3
Poly(propylene oxide) glycols are utilized extensively as soft segments in urethane foams, which, among
other applications, are used as automobile seats. It is frequently necessary to modify the growing species
in propylene oxide polymerization with ethylene oxide in order to produce a polymer with acceptable
reactivity with isocyanates and urethane product with desirable properties.
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55
POLYMERIZATION MECHANISMS
Table 2.4 Examples of Commercially Important Ring-Opening Polymerizations
Monomer
Polymer
Mechanism
O
CH2
CH2
O
O
CH2
O
n
CH2
trioxane
Cationic
polyoxymethylene
O
CH2
CH2
CH2
ethylene oxide
CH2
O
n
Anionic, cationic, coordination
poly(ethylene oxide)
O
CH2
CH2
CH2
CH2
(CH2)4
O
n
poly(tetramethylene oxide)
tetrahydrofuran
Cationic
O
CH2
NH
CH2
CH2
CH2
CH2
H
O
N
Caprolactam
(CH2)5
C
n
Hydrolytic, anionic
Polycaprolactam
(nylon 6)
O
C
CH2
O
CH2
CH2
CH2
CH2
O
O
caprolactone
Si
O
n
Anionic, cationic
CH3
O
Si
C
polycaprolactone
Si
O
(CH2)5
O
Si
Dimethysiloxane
(cyclic tetramer)
Si
CH3
O
n
poly(dimethylsiloxane)
Anionic, cationic
From McGrath, J. E., Makromol. Chem. Macromol. Symp., 42/43, 69, 1991. With permission.
Copyright 2000 by CRC Press LLC
56
POLYMER SCIENCE AND TECHNOLOGY
B. EPOXY RESINS
Epoxy resins are normally prepared by the base-catalyzed reaction between an epoxide such as epichlorohydrin and a polyhydroxy compound such as bisphenol A:
CH3
HO
O
C
OH + CH2
CH
CH2Cl
CH3
bisphenol A
epichlorohydrin
CH3
HO
OH
C
CH2
O
CH2
O
CH2
CH
CH2Cl •
NaOH
-NaCl
CH3
CH3
HO
O
C
CH
(2.52)
CH2
CH3
The molar ratio of epichlorohydrin to bisphenol A can range from as high as 10:1 to as low as 1.2:1.
This produces resins ranging from liquid to semisolid to solid and varying molecular weights and
softening points. The products are oligomers or prepolymers, which are hardly used as such; they have
pendant hydroxyl groups and terminal epoxy groups. The epoxy prepolymer can be cross-linked or cured
by reaction with a number of reagents, including primary and secondary amines.
C. POLYCAPROLACTAM (NYLON 6)
The industrial manufacture of nylons involves either water-initiated (hydrolytic) or a strong base-initiated
(anionic) polymerization of caprolactam. Polymerization by cationic initiation is also known, but monomer conversion and attainable molecular weights are inadequate for practical purposes and as such there
is no commercial practice of this process.
Hydrolytic polymerization of caprolactam is the most important commercial process for the production of nylon 6. The following synthetic scheme outlines hydrolytic polymerization of caprolactam:
250°C
HN
(CH2)5
C = O + H2O
250°C
(CH2)5
H2N
H
H2N
(CH2)5
COOH
H
N
H3N+ (CH2)5
COOH
-
COO
(2.53)
O
(CH2)5
C
OH + n-1H2O
(2.54)
Water opens the caprolactam ring producing aminocaproic acid, which is believed to exist as the
zwitterion. The zwitterion interacts with and initiates the step polymerization of the monomer, with the
ultimate generation of linear polymer molecules. In other words, the polymerization process involves
an initial ring-opening of the monomer that is followed by the step polymerization.
The hydrolytic polymerization process may be carried out as a continuous operation or it may be
operated batchwise. It involves heating caprolactam in an essentially oxygen-free atmosphere in the
presence of water to temperatures in the range 250 to 270°C for periods ranging from 12 h to more than
24 h. Most of the water used to initiate the reaction is removed during the process after about 80 to 90%
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57
POLYMERIZATION MECHANISMS
conversion. The overall polymerization involves various equilibria and does not result in complete
conversion of the caprolactam. The quantity of the residual monomer depends on the reaction temperature, which under industrial conditions amounts to 8 to 10%. In addition, there are about 3% of
predominantly cyclic low-molecular-weight oligomers. These impurities adversely affect the subsequent
processing and end-use performance of the polymer and must, therefore, be removed from it. This is
achieved either by hot-water extraction or by vacuum evaporation.
The second approach for the commercial synthesis of nylon 6, which accounts for up to 10% of the
volume of the polymer, is the base-initiated anionic polymerization of caprolactam. A small but important
number of applications utilize this process, which is characterized by high conversion rates. It involves
two techniques: “high-temperature” and “low-temperature” polymerizations. The high-temperature polymerization is carried out at temperatures above the melting point of nylon 6 (i.e., 220°C); whereas the
low temperature polymerization involves temperatures in the range 140 to 180°C, which are above the
melting point of caprolactam but below the melting point of the resulting polycaprolactam. The polymerization catalysts are strong bases such as sodium hydride or a Grignard reagent. To obtain satisfactory
low-temperature polymerization, a coinitiator such as N-acyl caprolactam or acyl-urea is employed in
addition to the strong base.
Sodium or magnesium caprolactam salt is produced by reaction of sodium hydride or a Grignard
reagent with caprolactam. A rapid polymerization occurs when the acylated lactam, the catalyst, and
monomer are mixed at temperature at or greater than 140°C. This polymerization is usually carried out
in a two-stream reactor in which one stream contains the catalyst dissolved in the monomer and the
second stream contains the initiator dissolved separately in the monomer:
Stream 1:
O
O
C
C
HN
+ NaH
( < 1 mol%)
100°C
dry
Na+N-
H2
(2.55)
Homogeneous Solution
(2.56)
+
1/2
(CH2)5
(CH2)5
monomer
catalyst
Stream 2:
O
O
CH3
C
C
O
N
(CH2)5
(CH2)5
initiator
Monomer + Catalyst + Initiator
100°C
C + NH
monomer
∆
>140°C
CH3
O
H
C
N
O
(CH2)5
C
O
N
C
(2.57)
n
(CH2)5
Unlike the high-temperature process where about 8 to 10% cyclics are generated, the equilibrium
monomer content of nylon 6 resulting from polymerization at temperature lower than 200°C (i.e., lowtemperature polymerization) can be less than 2%. The polymer therefore does not usually require any
additional purification. Also, the maximum rate of crystallization of nylon 6 falls within the range of
Copyright 2000 by CRC Press LLC
temperature employed in low-temperature polymerization. Consequently, the resulting polymers are
characterized by a high degree of crystallinity. Nylon 6 objects of any desired shape are obtainable in
essentially a single-stage polymerization. Therefore, this process is frequently referred to as “cast nylon 6”
or, in current usage, Reaction Injection-Molded (RIM) Nylon 6.
Example 2.4: Ethylene oxide polymerizes readily to high conversions under either anionic or cationic
conditions. Tetrahydrofuran can be induced to polymerize in the presence of phosphorous or antimony
pentafluorides as catalysts. Tetrahydropyran is unreactive under polymerization conditions. Explain these
observations.
Solution:
O
O
O
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
(Str. 14)
CH2
ethylene oxide
tetrahydrofuran
tetrahydropyran
From the structures of these compounds, it is apparent that the ease of polymerization is related to the
degree of ring strain. Ethylene oxide is a highly strained three-membered ring. The need to release this
strain provides the driving force for polymerization and is reflected in the ease with which ethylene
oxide undergoes polymerization. The ring strain decreases in the five-membered tetrahydrofuran and is
absent in the six-membered tetrahydropyran.
VI.
2.1.
PROBLEMS
Explain the following observations:
a. Butadiene polymerizes much less readily than vinyl acetate.
b. Increasing initiator concentration usually leads to lower average molecular weight.
2.2.
The use of phthalic anhydride along with maleic anhydride reduces the brittleness of resin described
in Example 2.2 in the text. Why?
2.3.
Melt polymerization of useful aliphatic polypyromellitimides is limited to those polyimides with melting
points sufficiently low that they remain molten under the polymerization conditions. Which of the two
diamines (ethylenediamine or decamethylenediamine) is more likely to form a useful polymer?
2.4.
Polyheteroaromatics can be produced from aromatic amino carboxylic acids with the following general
structure:
H2N
NH2
(Str. 15)
Ar
ROOC
COOR
Comment on the relative thermal stabilities of the polymers made from compounds where Ar is either
A, B, or C.
CH2
(A)
Copyright 2000 by CRC Press LLC
(B)
(C)
(Str. 16)
59
POLYMERIZATION MECHANISMS
2.5.
Several higher homologs of phenol such as p-tertiary butyl phenol, p-tertiary amyl phenol and p-tertiary
octyl-phenol are used to make oil-soluble phenolic resins for varnishes and other coatings systems.
What structural features of these compounds make oil solubility of the resins possible?
2.6.
Sketch, on the same graph, the molecular weight conversion curves for A, free-radical polymerization;
B, condensation polymerization; and C, living polymerization. Explain the basis of your sketch.
REFERENCES
1. Billmeyer, F.W., Jr., Textbook of Polymer Science, 3rd ed., Interscience, New York, 1984.
2. Prane, J.A., Introduction to Polymers and Resins, Federation of Societies for Coatings Technology, Philadelphia, 1986.
3. Allcock, H.R. and Lampe, F.W., Contemporary Polymer Chemistry, 2nd ed., Prentice-Hall, Englewood Cliffs, NJ,
1990.
4. Reimschuessel, H.K., J. Polym. Sci.: Macromol. Rev., 12, 65, 1977.
5. Handerson, J.F. and Szwarc, M., The Use of Living Polymers in the Preparation of Polymer Structures of Controlled
Architecture.
6. Sroog, C.E., J. Polym. Sci. Macromol. Rev., 11, 161, 1976.
7. Noren, G.K. and Stille, J.K., J. Polym. Sci. (D) Macromol. Rev., 5, 385, 1971.
8. Braunsteiner, E.E., J. Polym. Sci. Macromol. Rev., 9, 83, 1974.
9. Webster, O.W., Science, 251, 887, 1991.
10. McGrath, J.E., Makromol. Chem. Macromol. Symp., 42/43, 69, 1991.
11. Fried, J.R., Plast. Eng., 38(6), 49, 1982.
12. Fried, J.R., Plast. Eng., 38(10), 27, 1982.
13. Fried, J.R., Plast. Eng., 38(12), 21, 1982.
14. Fried, J.R., Plast. Eng., 39(3), 67, 1983.
15. Fried, J.R., Plast. Eng., 39(5), 35, 1983.
16. Chruma, J.L. and Chapman, R.D., Chem. Eng. Prog., p. 49, January 1985.
Copyright 2000 by CRC Press LLC
Chapter 3
Chemical Bonding and Polymer Structure
I.
INTRODUCTION
You will recall from your elementary organic chemistry that the physical state of members of a homologous series changes as the molecular size increases. Table 3.1 briefly outlines this for members of the
alkane series with the general formula [CnH2n+2]. From Table 3.1, it is obvious that moving from the
low- to the high-molecular-weight end of the molecular spectrum, members of the series change progressively from the gaseous state through liquids of increasing viscosity (decreasing volatility) to low
melting solids and ultimately terminate in high-strength solids. Polymers belong to the high-molecularweight end of the spectrum. In the following discussion, we will attempt to illustrate how the unusual
properties of high polymers are developed. To do this, it will be convenient to consider the chemical
and structural aspects of polymers at three different levels:
1. The chemical structure (atomic composition) of the monomer (primary structure)
2. The single polymer chain (secondary level)
3. Aggregation of polymer chains (tertiary structure)
But before we proceed into extensive discussion of these aspects, we must first consider the molecular
forces operative in polymers. After all, this is fundamental to understanding polymer structures.
II.
CHEMICAL BONDING
The electronic structure of atoms determines the type of bond between the atoms concerned. As we said
earlier, chemical bonds may be classified as primary or secondary depending on the extent of electron
involvement. Valence electrons are involved in the formation of primary bonds. This results in a substantial lowering of the potential energies. Consequently, primary bonds are quite strong. On the other
hand, valence electrons are not involved in the formation of secondary bonds — leading to weak bonds.
Primary and secondary bonds can be further subdivided:
1. Primary bonds
a. Ionic
b. Covalent
c. Metallic
2. Secondary bonds
a. Dipole
b. Hydrogen
c. Induction
d. van der Waals (dispersion)
We now discuss briefly some of these bonds that occur in polymers.
A. THE IONIC BOND
The so-called inert gases — Ne, Ar, and Kr — have completely filled s and p outermost orbitals, resulting
in a spherical distribution of electrons. The inertness of these elements (gases) suggests that their
electronic configuration confers stability. It is indeed observed that all elements seek to achieve this
stable inert gas electronic configuration. They do this by either losing, gaining, or sharing electrons.
The mutual satisfaction of the need to attain the inert gas electronic configuration by those elements
that lose electrons (electropositive elements) and those that gain electrons (electronegative elements)
leads to the ionic bond. To illustrate this, consider sodium chloride [NaCl]. Sodium (with low ionization
energy) can easily lose the outermost 3s electron to achieve the stable inert gas configuration. Chlorine,
0-8493-????-?/97/$0.00+$.50
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POLYMER SCIENCE AND TECHNOLOGY
Table 3.1 Change of State with Molecular Size for the Alkane
[CnH2n+2] Series
No. of
Carbon Atoms
Molecular State
1
2–4
5–10
10–102
102–103
103–106
Methane — boiling point –162°C
Natural gas — liquefiable
Gasoline, diesel fuel — highly volatile, low viscosity liquid
Oil, grease — nonvolatile, high viscosity liquid
Wax — low melting solid
Solid — high strength
on the other hand (with large electron affinity), can achieve a stable electronic configuration by gaining
an extra electron. The loss of an electron by sodium results in a positively charged sodium ion, while
the gain of an extra electron by the chlorine atom results in a negatively charged chloride ion:
Na + Cl → Na + + Cl −
(Str. 1)
The bonding force in sodium chloride is a result of the electrostatic attraction between the two ions.
Ionic bonds are not common features in polymeric materials. However, divalent ions are known to
act as cross-links between carboxyl groups in natural resins. The relatively new class of polymers known
as ionomers contain ionic bonds, as will be discussed later.
B. THE COVALENT BOND
In the previous section, we saw that the stable inert gas electronic configuration can be achieved by
electropositive elements through ionization. For elements in the central portion of the periodic table,
ionic bonding is impossible because a large amount of energy would be required to ionize the valence
electrons. However, stable electronic configuration can be attained by the sharing of valence electrons.
Bonds formed by electron sharing are called covalent bonds. Consider the formation of methane from
hydrogen and carbon. The carbon atom has four unpaired electrons in its outer electron shell, while
hydrogen has one electron. By sharing electrons, one from each atom per bond, a stable octet is obtained
for the carbon atom and a stable pair for each hydrogen atom. The result is the methane molecule:
••
••
C + 4H
H
••
H C H
••
H
H
or
H C H
(Str. 2)
H
This is the predominant bond in polymers. Covalent bonds can be single, double, or triple depending
on the number of electron pairs. Typical values of bond strengths, expressed as dissociation energy, for
covalent bonds that commonly occur in polymers are summarized in Table 3.2.
Recall that dissociation energy is that required to break the bond. It has a direct relationship with the
thermal stability of polymers. Note also that while atoms are free to rotate about single bonds (flexible),
they remain spatially fixed (rigid) for double and triple bonds.
C. DIPOLE FORCES
Molecules are electrically neutral, but will have a permanent dipole if the centers of the positive and
negative charges do not coincide; this arises if the electrons shared by two atoms spend more time on
one of the atoms due to differences in electronegativity. This can be illustrated by considering a diatomic
molecule such as hydrogen chloride, HCl. Because chlorine is more electronegative than hydrogen, the
shared pair of electrons between the chlorine atom and the hydrogen atom is drawn closer to the chlorine
atom. Consequently, the chlorine atom has net negative charge while the hydrogen atom has a net positive
charge:
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CHEMICAL BONDING AND POLYMER STRUCTURE
Table 3.2 Properties of Some Primary Covalent Bonds in Polymers
Type of
bond
C–C
C› C
C› C
C–H
C–O
C› O
C–N
C› N
C› N
C–S
Bond length
in Å
Average dissociation
energy (kcal/mol)
Type of
bond
Bond length
in Å
Average dissociation
energy (kcal/mol)
1.54
1.34
1.20
1.09
1.43
1.23
1.47
1.27
1.16
1.81
83
147
194
99
84
171
70
147
213
62
C› S
C–Cl
N–H
N–O
N–Si
O–H
O–O
O–Si
S–S
S–H
1.71
1.77
1.01
1.15
1.74
0.96
1.48
1.64
2.04
1.35
124
79
93
57
—
111
33
88
51
81
Figure 3.1 Dipole–dipole interaction between polar molecules.
δ+
H
δCl
(Str. 3)
Any diatomic molecule in which there is a separation of positive and negative charge is said to be polar.
As we shall see in Section III.A, in molecules containing more than two atoms the polarity of the
molecule is determined by the bond angles. Polar molecules therefore have a small separation of charge,
and this sets up a permanent dipole. Dipoles interact through coulombic forces, which can become quite
significant at molecular distances. Polar molecules are held together in the solid state by the interaction
between oppositely charged ends of the molecules. The interaction forces between these molecules are
called dipole-interaction forces or dipole–dipole interaction (Figure 3.1). This type of molecular orientation is generally opposed by thermal agitation. Consequently, the dipole–dipole interaction is temperature dependent. As we shall see later, dipole forces play a significant role in determining the tertiary
structure and, hence, properties of some polymers.
D. HYDROGEN BOND
A particularly important kind of dipole interaction is the hydrogen bond. This is the bond between a
positively charged hydrogen atom and a small electronegative atom like F, O, or N. The anomalous
properties of water, for example, are associated with the hydrogen bonding between water molecules
(Figure 3.2). The difference in electronegativities between hydrogen (2.1) and oxygen (3.5) causes the
bonding electrons in H2O to shift markedly to the oxygen atom so that the hydrogens behave essentially
as bare protons. Hydrogen bonding is limited primarily to compounds containing F, N, and O because
the small size of hydrogen permits these atoms to approach the hydrogen atom in another molecule very
closely. For example, in spite of the similarity in electronegativities between Cl and N (3.0 for both),
HCl with the larger chlorine atoms shows hardly any tendency to form hydrogen bonds.
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POLYMER SCIENCE AND TECHNOLOGY
Figure 3.2 Hydrogen bonding between two water molecules.
Table 3.3 Relative Interaction Energies
for Different Types of Bonds Found in Polymers
Nature of Interaction
Dipole–induced dipole
van der Waals
Dipole–dipole
Hydrogen bond
Covalent bond
Ionic bond
Interaction Energy (kJ/mol)
≤2
0.08–4.0
≤20
≤50
60–600
560–1000
Hydrogen bonds are relatively stronger than dipole bonds due to the small size of the hydrogen ion
(Table 3.3). In polymers, hydrogen bonding usually occurs between functional groups in the same or
different molecules. The hydrogen is generally part of such groups as carboxyl, hydroxyl, amine, or
amide, while the other atom in the hydrogen bond is frequently oxygen (in carbonyl, ethers, or hydroxyls)
or nitrogen (in amines, amides, urethanes, urea). The hydrogen bond plays a vital role in the structure
and properties of polymers, particularly proteins.
E. INDUCTION FORCES
Every dipole has an electric field associated with it. This electric field is capable of inducing relative
displacements of the electrons and nuclei in neighboring molecules. The result is that the surrounding
molecules become polarized, i.e., possess induced dipoles. Intermolecular forces, called induction forces,
exist between the permanent and induced dipole. Induction forces are weak and temperature independent.
The ease with which molecules can be polarized — referred to as polarizability — varies.
F. VAN DER WAALS (DISPERSION) FORCES
From the above, it would be expected that the gases He, Ne, Ar, and Kr are incapable of forming any
type of bonds (ionic, covalent, or metallic). In fact, these so-called inert gases derive the name from that
usual stability (considerable reluctance to undergo reactions). However, at sufficiently low temperature
these gases are known to condense to form solids. Similarly, molecules such as methane [CH4], carbon
dioxide [CO2], and hydrogen [H2] have all the valency requirements fulfilled and should, in principle,
be incapable of forming bonds. Yet, these also solidify at sufficiently low temperatures. It is therefore
apparent that some form of intermolecular force exists in these materials.
Electrons are usually in constant motion about their nuclei. At any particular instant, the centers of
negative charge of the electrons cloud may not coincide with those of the nuclei. Consequently, instantaneous (fluctuating) dipoles exist even in nonpolar materials. If the orientations of fluctuating dipoles
in neighboring molecules are in proper alignment, intermolecular attractions occur. These attractive
forces are referred to as van der Waals (dispersion) forces. Van der Waals forces are present in all
molecules and, as we shall see later, they contribute significantly to the bonding in polymers. Table 3.3
shows the relative magnitudes of the different interaction energies, while typical melting points for
various compounds are shown in Table 3.4.
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CHEMICAL BONDING AND POLYMER STRUCTURE
Table 3.4 Typical Melting Temperatures for Some
Substances with Different Types of Chemical Bonding
Type of Bond
Ionic
Covalent
Metallic
Van der Waals
Hydrogen
Substance
Melting Temperature (°C)
Na F
NaCl
Na Br
Na I
NaO
CaO
Sr O
Ba O
Al2O3
Ge
GaAs
Si
SiC
Diamond
Na
Al
Cu
Fe
W
Ne
Ar
CH4
Kr
CL2
HF
H2O
988
801
740
660
2640
2570
2430
1923
3500
958
1238
1420
2600
3550
98
660
1083
1535
3370
–249
–189
–184
–157
–103
–92
0
Example 3.1: Explain the trend in the melting points of the following:
Compound/Element
Melting Point
KF
Na
F2
Polyethylene
46
97.5
–219.6
135
Solution:
Compound/Element
KF
Na
F2
Polyethylene (PE)
Chemical Bonding
Bond Type
Ionic
Metallic
van der Waals
van der Waals
Primary
Primary
Secondary
Secondary
Primary bonds are stronger than secondary bonds. Within the primary bonds ionic bonds are generally
stronger than metallic bonds, particularly for univalent metals. Both fluorine and PE molecules are held
by van der Waals forces. In the case of fluorine molecules, these forces are readily overcome by thermal
agitation and, consequently, fluorine is a gas at room temperature. However, because of the macromolecular sizes of PE molecules, in the aggregate the van der Waals forces become very large. This,
coupled with extensive physical entanglements, results in a high melting point.
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POLYMER SCIENCE AND TECHNOLOGY
III.
PRIMARY STRUCTURE
Primary structure refers to the atomic composition and chemical structure of the monomer — the building
block of the polymer chain. An appreciation of the nature of the monomer is fundamental to understanding
the structure–property relationship of polymers. The chemical and electrical properties of a polymer are
directly related to the chemistry of the constituent monomers. The physical and mechanical properties
of polymers, on the other hand, are largely a consequence of the macromolecular size of the polymer,
which in itself is related to the nature of the monomer. By definition, a polymer is a chain of atoms
hooked together by primary valence bonds. Therefore, basic to understanding the structure of the
monomer vis-á-vis the structure and properties of the resulting polymer is a fundamental understanding
of:
•
•
•
•
The nature of bonds in monomers (chemical bonding)
The type of monomers that are capable of forming polymers (functionality of monomers)
The mode of linking of monomers (polymerization mechanisms)
The chemical composition of monomers and the properties conferred on monomers as a result of their
chemical composition
We have discussed chemical bonding, monomer functionality, and polymerization mechanisms in
previous sections. Our attention now focuses on the chemical composition of monomers.
A. POLARITY OF MONOMERS
The chemical composition and atomic arrangement of an organic molecule confer certain properties on
the molecule. One such property is the polarity of the molecule. We now discuss this briefly.
The ionic compound sodium chloride is formed by an electron transfer from sodium (leaving behind
a positively charged ion) to chlorine (leaving a negatively charged chloride ion). A diatomic molecule
with such a pair of equal but opposite charges possesses a permanent dipole moment and is said to be
polar. Sodium chloride, like all ionic substances where complete charge transfer has occurred, is highly
polar. This polarity is responsible for the electrostatic attraction between adjacent ions in solid sodium
chloride.
Covalent molecules, on the other hand, are formed by the sharing of electrons between the constituent
atoms. In a diatomic molecule formed from two like atoms (e.g., H2), the electron pair linking the two
atoms is equally shared and the molecule is said to be nonpolar. But when molecules are formed from
two unlike atoms (e.g., hydrogen fluoride, HF), the distribution of the electron cloud is concentrated on
the more electronegative atoms (fluorine, in this case). Here again, as in ionic compounds, there is a
separation of positive and negative charge and the molecule is said to be polar. However, since no
complete charge transfer has taken place in this case, the polarity (of covalent molecules) is less than
that of ionic compounds Even among covalent molecules, the degree of polarity varies depending on
the electronegativities (electron-attracting ability) of the constituent atoms. The electronegativities of
atoms commonly occurring in organic molecules are shown in Table 3.5. It is evident from the table
that groups like C–Cl, C–F, –CO–, –CN, and –OH are polar.
In a polyatomic molecule, the polarity is a vector sum of all the dipole moments of the groups within
the molecule. This depends on the spatial distribution (symmetry) of the groups within the molecule.
To illustrate this, let us consider two triatomic molecules: water [H2O] and carbon dioxide [CO2]. Both
the OH and CO groups are polar. But while the H2O molecule is polar, CO2 is a nonpolar molecule.
The structure of CO2 is linear, resulting in a cancellation of the dipole moments. However, H2O has a
triangular structure and, consequently, possesses an overall dipole moment (Figure 3.3).
Carbon tetrachloride, CCl4, is another molecule that is nonpolar even though it has four polar C–Cl
bonds. The nonpolar nature of CCl4 is due to the symmetrical distribution of the four chlorines around
the carbon atom. Replacement of one of the chlorine atoms by hydrogen destroys symmetry. The resulting
Table 3.5 Electronegativities of Some Elements
Atom
Electronegativity
H
2.1
C
2.5
N
3.0
O
3.5
F
4.0
Sl
1.8
S
2.5
Cl
3.0
From Pauling, L., The Nature of the Chemical Bond, Cornell University
Press, Ithaca, NY, 1960. With permission.
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65
CHEMICAL BONDING AND POLYMER STRUCTURE
Figure 3.3 Effect of symmetry on polarity of molecules.
molecule, chloroform [CHCCl3], is polar. Monomers such as ethylene and propylene are nonpolar, and
so are the polymers derived from them. On the other hand, the polar monomers vinyl chloride [CH2› CHCl]
and acrylonitrile [CH2› CHCN] result in polar polymers. However, the symmetrical monomers vinylidene
chloride and vinylidene fluoride lead to nonpolar polymers (Table 3.6). The characteristic interunit
linkages in condensation polymers, for example, –CO.O– (ester), –CO.NH– (amide), –HN–CO–NH–
(urea), and –O.CO–NH– (urethane) are polar. Polarity, as we shall see later, affects the intermolecular
attraction between chain molecules, and thus the regularity and symmetry of polymer structure. Naturally,
properties such as the solubility and electrical nature of polymers, which depend on polymer structure,
are intimately related to polarity.
IV.
SECONDARY STRUCTURE
To be able to understand polymer properties, we must be able to develop a physical picture of what
these long molecules are really like. This is what we refer to as the secondary structure, i.e., the size
and shape of an isolated single molecule. The size of the polymer is best discussed in terms of molecular
weight. The shape of the polymer molecule (molecular architecture) will be influenced naturally by the
nature of the repeating unit and the manner in which these units are linked together. It is therefore
convenient to consider polymer shape in two contexts:
• Configuration — Arrangement fixed by primary valence bonds; can be altered only through the breaking
or reforming of chemical bonds
• Conformation — Arrangement established by rotation about primary valence bonds
A. CONFIGURATION
As we saw earlier, a polymer molecule may be linear, branched, or cross-linked depending on the
functionality of the monomers used. But let us look more closely at the polymer chain. If repeating units
along the chain are chemically and sterically regular, then the polymer is said to possess structural
regularity. To consider structural regularity, we need to define two terms: recurrence regularity and
stereoregularity.
Recurrence regularity refers to the regularity with which the repeating unit occurs along the polymer
chain. This may be illustrated by examining the polymers resulting from monosubstituted vinyl monomers. Here there are three possible arrangements:
• Head-to-tail configuration
CH2
CH
CH2
X
CH
CH2
X
CH
CH2
X
CH —
(3.1)
X
• Head-to-head configuration
CH2
CH
X
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CH2
CH
CH
X
X
CH2
(3.2)
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POLYMER SCIENCE AND TECHNOLOGY
Table 3.6 Polarity of Monomers and Their Associated Polymers
Monomer
CH2
Polarity
CH2
Polymer
Nonpolar
CH2
Ethylene
CH2
CH2
Nonpolar
CH2
CH
Polar
CH2
CCl2
Nonpolar
CH
Cl
CH2
Polar
Nonpolar
C
Cl
Poly(vinylidene chloride)
CH
Polar
CH2
CN
Acroylonitrile
CF2
Nonpolar
Cl
Poly(vinyl chloride)
Vinylidene chloride
CH2
CH2
CH3
Polypropylene
Cl
Vinyl chloride
CH2
Nonpolar
Polypropylene
CH3
Propylene
CH2
CH2
Polarity
CN
Polar
CN
Polyacrylonitrile
CF2
Nonpolar
CF2
Tetrafluoroethylene
(symmetrical)
CF2
Nonpolar
Polytetrafluorethylene
(Teflon)
• Tail-to-tail configuration
CH2
CH
CH
X
X
CH2
CH2
CH
(3.3)
X
The last two configurations do not appear in any measurable extent in known polymers.
Stereoregularity refers to the spatial properties of a polymer molecule. To discuss this, let us consider
two examples.
1. Diene Polymerization
You will recall that the propagation step in the polymerization of diene monomers (monomers with
two double bonds) can proceed by either of two mechanisms: 1,2, and 1,4 additions. In 1,2 addition
the resulting polymer unsaturation is part of the pendant group, while in 1,4 addition the unsaturation
is part of the backbone. In the latter case, the backbone has a rigid structure and rotation is not free
around it. Therefore, two different configurations, known as cis and trans, are possible. For example,
1,4-polyisoprene:
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CHEMICAL BONDING AND POLYMER STRUCTURE
CH2
CH2
C
(Str. 4)
C
CH3
H
cis-1,4-polyisoprene (natural rubber)
CH2
H
C
(Str. 5)
C
CH3
CH2
trans-1,4-polyisoprene (gutta-percha)
2. Tacticity
Polymers of monosubstituted olefins [CH2› CHXl contain a series of asymmetric carbon atoms along
the chain. For this type of polymers, in a planar zigzag form, three arrangements are possible, namely:
• Isotactic — All the substituent groups, R, on the vinyl polymer lie above (or below) the plane of the
main chain.
R
R
R
R
R
(Str. 6)
• Syndiotactic — Substituent groups lie alternately above and below the plane.
R
R
R
(Str. 7)
R
R
• Atactic — Random sequence of position of substituent occurs along the chain.
R
R
R
R
R
(Str. 8)
B. CONFORMATION
In addition to the molecular shape fixed by chemical bonding, variations in the overall shape and size
of the polymer chain may occur due to rotation about primary valence bonds (conformation). A polymer
molecule may assume a large or limited number of conformations depending on:
• Steric factors
• Whether the polymer is amorphous or crystalline
• Whether the polymer is in a solution state, molten state, or solid state
To amplify the discussion, let us consider the possible arrangements of a single isolated polymer chain
in dilute solution. We start with a short segment of the chain consisting of four carbon atoms (Figure 3.4).
Figure 3.4 A segment of polymer chain showing four successive
chain atoms; the first three of these define a plane, and the fourth
can lie anywhere on the indicated circle which is perpendicular to
and dissected by the plane.
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POLYMER SCIENCE AND TECHNOLOGY
Figure 3.5 The fully extended all-trans conformation of a carbon–carbon chain.
We define a plane by three of the carbon atoms in this segment and allow free rotation about the
carbon–carbon bond. In this case, the fourth carbon atom can be anywhere on the circle indicated in the
figure. Because of steric hindrance, some positions will certainly be more probable than others. Each
successive carbon atom on the chain can similarly take any of the several positions in a circle based,
randomly, on the position of the preceding atom. For a chain consisting of thousands of carbon atoms,
it can thus be seen that the number of conformations is literally infinite. One of these conformations of
particular interest is that in which each successive carbon atoms lies in the same plane in the trans
location with respect to earlier carbon atoms in the chain — thus forming a fully extended plane of
zigzag arrangement of carbon atoms (Figure 3.5).
This represents one of the two extreme shapes of a polymer chain, the other being the completely
random coil. The planar zigzag conformation exists in some crystalline polymers or in highly oriented
amorphous polymers. Typical examples are simple molecules like PE, PVC, and polyamides, where the
small size of the pendant group does not complicate alignment and packing. In those polymers with
large and bulky side groups like PP and PS (in general, isotactic and syndiotactic polyolefins), it is
impossible sterically to accommodate the pendant groups in the planar zigzag. Consequently, the entire
main chain is rotated in the same direction to form either a right- or left-handed helix. This occurs
exclusively in the crystalline form of stereoregular polymers with bulky side groups (Figure 3.6).
The other extreme of the conformation spectrum that may be assumed by the polymer chain is the
completely random coil. Polymers that are in solution, in melt, or amorphous in the solid state assume
this conformation. Between these two extremes (planar zigzag and random coil conformation) the number
of conformation shapes that a polymer chain can assume is virtually limitless. This, of course, assumes
that there is free rotation about single bonds. In practice, however, there is no such thing as completely
free rotation. All bonds have to overcome certain rotational energy barriers whose magnitude depends
on such factors as steric hindrance, dipole forces, etc. (Figure 3.7).
The thermal energy of the molecular environment provides the energy required to overcome the
rotational energy barrier. Consequently, the shape (flexibility) of a polymer molecule is temperature
dependent. At sufficiently high temperatures, the polymer chain constantly wiggles, assuming a myriad
of random coil conformations. As we shall see later, the flexibility of polymer molecules, which is a
function of substituents on the backbone, has a strong influence on polymer properties.
C. MOLECULAR WEIGHT
The terms giant molecule, macromolecule, and high polymer are used to describe a polymer molecule
to emphasize its large size. We noted earlier that the same bonding forces (intra- and intermolecular)
operate in both low- and high-molecular-weight materials. However, the unique properties exhibited by
polymers and the difference in behavior between polymers and their low-molecular-weight analogs are
attributable to their large size and flexible nature.
Important mechanical properties (tensile and compressive strengths, elongation at break, modulus,
impact strength) and other properties (softening point, solution and melt viscosities, solubility) depend
on molecular weight in a definite way. At very low molecular weights, hardly any strength, for example,
is developed. Beyond this MW or DP, there is a steep rise in the performance until a certain level, beyond
which the properties change very little with increase in molecular weight. Finally, an asymptotic value
is reached (Figure 3.8). The curve in Figure 3.8 is general for all polymers. Differences exist only in
numerical details. Optical and electrical properties, color, and density show a less marked dependence
on molecular weight.
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CHEMICAL BONDING AND POLYMER STRUCTURE
Figure 3.6 Helical conformations of isotactic vinyl polymers. (From Gaylord, N.G. and Mark, H., Linear and
Stereoregular Addition Polymers, Interscience, New York, 1959. With permission.)
Example 3.2: Which of the following materials will be most suitable for the manufacture of thermoplastic sewage pipe? Explain your answer very briefly.
(
CH2
CH2
)
50
(
CH2
A
CH2
)
5000
( CH2
B
CH2
C
)
500,000
(Str. 9)
Solution:
Material
Molecular Weight
A
B
C
1.4 × 103
1.4 × 105
1.4 × 107
Material B will be most suitable. The molecular weight of A is too low, and the material will not have
developed the physical properties necessary to sustain the mechanical properties that a plastic pipe must
withstand. On the other hand, the molecular weight of material C is relatively too high to permit easy
processing.
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POLYMER SCIENCE AND TECHNOLOGY
Figure 3.7 Rotational energy as a function of substitution and interaction of substituent groups.
We also noted earlier that irrespective of the polymerization mechanism, the formation of polymer
is a purely random occurrence. Consequently, unlike biological systems, synthetic polymers do not
consist of identical molecules, but represent a mixture of many systems each of which has a different
molecular weight. In order to characterize polymers, therefore, we use the molecular weight distribution
(MWD) curve, which represents a plot of the percentage (frequency) of a particular species against its
molecular weight (Figure 3.9).
As a result of the existence of different sizes of molecular species in a polymeric material, we cannot
strictly speak of the molecular weight of a polymer. Instead we use molecular weight averages to express
the size of synthetic polymers. Different average molecular weights exist. The most common ones in
use are number-average molecular weight, Mn, and weight-average molecular weight, Mw . Others are
the z-average molecular weight, Mz, and viscosity-average molecular weight, Mv . Below are the relevant
formulas for computing these average molecular weights (Equations 3.4–3.7).
∞
∞
Mn −
W
∞
∑N
i =1
Copyright 2000 by CRC Press LLC
=
i
∑
NiMi
i =1
∞
∑N
i =1
i
=
i
∑W
∞
i =1
∑W
i
i =1
Mi
=
1
∑w
i
(3.4)
Mi
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CHEMICAL BONDING AND POLYMER STRUCTURE
∞
∞
Mw =
∑w M =
i
∑
∞
wi M i
W
i =1
i
=
i =1
i
∑N M
2
i
i =1
∞
∑N M
i
(3.5)
i
i =1
Mv =
∞
∑
i =1
w i M ai
1a
=
∑
N i M i3
∑
2
i
where Ni
W
N
wi
Wi
a
=
=
=
=
=
=
∑
1a
(3.6)
NiM
∑w M
2
i
i =1
∞
∑
(3.7)
wi M i
i =1
number of molecules having molecular weight Mi
total weight
total number of molecules
weight fraction of molecules having molecular weight Mi
weight of molecule having molecular weight Mi
constant in Mark–Houwink equation (η) Kma in which the intrinsic viscosity (η) and the
molecular weight M are related through constants K and a for given polymer/solvent system;
a is at least 0.5 and mostly less than 0.8.
Figure 3.8 Change of physical properties
with molecular weight.
Copyright 2000 by CRC Press LLC
∑
i
=
i =1
∞
i =1
N i M ia +1
i =1
∞
NiMi
i =1
∞
∞
Mz =
∞
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POLYMER SCIENCE AND TECHNOLOGY
Figure 3.9 Molecular weight distribution
curve.
Example 3.3: The following data were obtained in a determination of the average molecular weight of
a polymer.
Molecular Weight
Weight (g)
60,000
40,000
20,000
10,000
1.0
2.0
5.0
2.5
a. Compute the number average and the weight average-molecular weights, Mn and Mw .
b. What is the polydispersity of the polymer and how many molecules are in 1 gram of polymer?
Solution:
a.
∑W
∑W M
i
i
Mn =
where Wi = weight species i.
i
1 + 2 + 5 + 2.5
1 60 × 103 + 2 40 × 103 + 5 20 × 103 + 2.5 10 4
= 18, 600 g mol
MW =
=
∑w M ;
i
i
wi =
Wi
= weight fraction of species i
W
10 4
[6 + 2 × 4 + 5 × 2 + 2.51]
10.5
= 25, 200 g mol
b. Polydispersity = Mw M n =
Copyright 2000 by CRC Press LLC
25, 200
= 1.35
18, 600
73
CHEMICAL BONDING AND POLYMER STRUCTURE
Molecules g = N I M n =
6.02 × 10 23
= 3.25 × 1019
18, 600
The molecular weight of polymers can be determined by a number of physical and chemical methods.
These include (1) end group analysis, (2) measurement of colligative properties, (3) light scattering,
(4) ultracentrifugation, (5) dilute solution viscosity, and (6) gel permeation chromatography (GPC). The
first four methods permit a direct calculation of molecular weight without the need to resort to calibration
by another method; that is, the methods are, in principle, absolute. The last two methods require proper
calibration to obtain the value of molecular weight. Colligative properties are determined by the following
measurements on dilute polymer solutions:
•
•
•
•
Vapor pressure lowering
Boiling point elevation (ebulliometry)
Freezing point depression (cryoscopy)
Osmotic pressure (osmometry)
The number-average weight, Mn, is observed from end-group analysis, colligative property measurements, and gel permeation chromotography. The weight-average molecular weight, Mw, is determined
from light scattering, ultracentrifugation and gel permeation chromatography. z-average molecular
weight, Mz, is determined from GPC, while viscosity-average molecular weight, Mv , can be determined
from measurements of polymer solution viscosity.
V. TERTIARY STRUCTURE
A given polymeric solid material is an aggregate of a large number of polymer molecules. Depending
on the molecular structure, the process of molecular aggregation occurs essentially by either of two
possible arrangements of molecules, leading to either a crystalline or amorphous material. However,
irrespective of the type of molecular arrangement, the forces responsible for molecular aggregation are
the intermolecular secondary bonding forces. The overall bonding energies due to secondary bonding
forces range from 0.5 to 10 kcal/mol compared with those of primary bonding forces, which are of the
order 50 to 100 kcal/mol. But when molecules are large enough, the attractive forces resulting from the
secondary intermolecular bonding forces may build up to such a level that, in some cases, they become
greater than the primary valence forces responsible for intramolecular bonds. The magnitude of these
secondary bonding forces, coupled with the high physical entanglement between chains, dictates many
polymer properties. Tertiary structure is concerned with the nature of the intermolecular secondary
bonding forces and with structural order of the resulting polymer.
A. SECONDARY BONDING FORCES (COHESIVE ENERGY DENSITY)
As we said earlier, secondary bonds consist of dipole, induction, van der Waals, and hydrogen bonds.
Dipole forces result from the attraction between permanent dipoles associated with polar groups. Induction forces arise from the attraction between permanent and induced dipoles, while van der Waals
(dispersion) forces originate from the time-varying perturbations of the electronic clouds of neighboring
atoms. Hydrogen bonds are very important in determining the properties of such polymers as polyamides,
polyurethanes, and polyureas. In general, the magnitude of the bond energies decreases from hydrogen
bond to dipole bond to van der Waals (dispersion) forces.
A quantitative measure of the magnitude of secondary bonding forces is the cohesive energy density
(CED), which is the total energy per unit volume needed to separate all intermolecular contacts and is
given by:
CED =
∆E v
VL
(3.8)
where ∆Ev = molar energy of vaporization
VL = molar volume of the liquid
It can be shown from the Classius–Clapeyron equation that
∆E v = ∆H v − RT
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(3.9)
74
POLYMER SCIENCE AND TECHNOLOGY
where ∆Hv = molar heat of vaporization
T = absolute temperature (K)
Consequently,
CED =
∆E v ∆H v − RT
=
VL
VL
(3.10)
For liquids of low molecular weight the energy necessary to separate molecules from one another is
evaluated from the heat of evaporation or from the dependence of vapor pressure on temperature. Since
polymers cannot be evaporated, the cohesive energy density is estimated indirectly by dissolution in
liquids of known cohesive energy density. To do this, we employ the relation between the cohesive
energy density and solubility parameter (Equation 3.11).
CED = δ 2
(3.11)
where δ = solubility parameter. As a first approximation and in the absence of strong interactions such
as hydrogen bonding, a polymer δ2 will dissolve in a solvent δ1 if
δ1 – δ2 ≤ 1.7 – 2.0.
Values of solubility parameters and cohesive energy of some polymers are given in Table 3.7. The value
of Ecoh is also dependent on the molar volume. For polymers the appropriate volume is that occupied
by each repeat unit in the solid state. Thus Ecoh represents the cohesive energy per repeat unit volume,
VR. These simple relations as stated before, however, are not exact; stronger interactions change the
validity of Equation 3.11. However, significant practical predictions can be made from the values in
Table 3.7, such as what solvents will dissolve a given polymer.
Table 3.7
Cohesive Energy of Polymers
d
(cal1/2/cm3/2)
Ecoh (from δ)
(cal/mol)
Polymers
From
To
VR
(cm3/mol)
From
Polyethylene
Polypropene
Polyisobutene
Polyvinylchloride
Polyvinylidene chloride
Polyvinyl bromide
Polyvinylfluoroethylene
Polychlorotrifluoroethylene
Polyvinyl alcohol
Polyvinyl acetate
Polyvinyl propionate
Polystyrene
Polymethyl acrylate
Polyethyl acrylate
Polypropyl acrylate
Polybutyl acrylate
Polyisobutyl acrylate
Poly-2,2,3,3,4,4,4, heptafluorobutyl acrylate
Polymethyl methacrylate
Polyethyl methacrylate
Polybutyl methacrylate
Polyisobutyl methacrylate
Poly-tert,butyl methacrylate
Polyethoxyethyl methacrylate
7.7
8.3
7.8
9.4
9.9
9.5
6.2
7.2
12.6
9.35
8.8
8.5
9.7
9.2
9.05
8.8
8.7
6.7
9.1
8.9
8.7
8.2
8.3
9.0
8.35
9.2
8.1
10.8
32.9
49.1
66.8
45.2
1,950
3,300
4,060
3,990
63.8
50.0
61.8
35.0
72.2
90.2
98.0
70.1
86.6
103.0
119.5
114.2
148.0
86.5
102.4
137.2
135.7
138.9
145.6
4,850
1,920
3,200
5,560
6,310
6,900
7,080
6,600
7,330
8,440
9,360
9,020
6,640
7,160
8,110
10,380
9,120
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7.9
14.2
11.05
9.3
10.4
9.4
9.1
11.0
19.8
9.15
9.0
10.5
9.9
11,790
To
2,290
4,160
4,300
5,270
3,860
7,060
8,820
8,400
7,500
7,650
9,900
14,420
14,170
8,570
11,110
14,960
9,570
15,270
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CHEMICAL BONDING AND POLYMER STRUCTURE
Table 3.7 (continued)
Cohesive Energy of Polymers
d
(cal1/2/cm3/2)
Polymers
From
To
Polybenzyl methacrylate
Polyacrylonitrile
Polymethacrylonitrile
Poly-a-cyanomethyl
Polybutadiene
Polyisoprene
Polychloroprene
Polyepichlorohydrin
Polyethylene terephthalate
Polyhexamethylene adipamide
Poly(δ-aminocaprylic acid)
Polyformaldehyde
Polytetramethylene oxide
Polyethylene sulfide
Polypropylene oxide
Polystyrene sulfide
Polydimethyl siloxane
9.8
12.5
10.7
14.0
0.1
7.9
8.2
9.4
9.7
13.6
12.7
10.2
8.3
9.0
7.5
9.3
7.3
10.0
15.4
Ecoh (from δ)
(cal/mol)
VR
(cm3/mol)
152.0
44.8
63.9
82.1
60.7
75.7
71.3
69.7
143.2
208.3
135.9
25.0
74.3
47.9
57.6
115.8
75.6
14.5
8.6
10.0
9.25
10.7
11.0
8.55
9.4
9.9
7.6
From
To
14,600
7,000
7,320
16,090
3,900
4,730
4,790
6,160
13,470
15,200
10,620
2,600
5,120
3,880
3,240
4,030
17,260
4,490
7,570
6,100
16,390
138,500
21,920
3,030
5,430
1,230
5,650
10,020
4,300
From Van Krevelen, D.W., Properties of Polymers, Elsevier, Amsterdam, 1972. With permission.
∆Hvap] of two solvents:
Example 3.4: The table below shows the density and enthalpy of vaporization [∆
methylethyl ketone and acetone.
Solvent
Density
(g/cm3)
∆Hv·
(cal/g)
0.8
0.8
106
125
Methylethyl ketone
Acetone
Which is a better solvent for polystyrene at room temperature? The CED for polystyrene is 75 cal/cm3.
Assume room temperature is 27°C.
Solution: Basis = 1 g of solvent*
CED =
∆H v′ − RT ∆H v⋅ − RT
=
VL
1 ρsol
= ρsol ( ∆H v − RT)
where ρsol = density of solvent. In the SI system
R = 8.303 × 103
Nm
(K)kg mol
= 8.303 × 103 = J (K) ( kg mol)
* Units of ∆Hv are in cal/g; therefore, the units of the term RT must be consistent with this.
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POLYMER SCIENCE AND TECHNOLOGY
R = 1.984 cal (K) (g mol)
= 1.984 cal (g) ; T = 27 + 273 = 300 K
Recall 1 cal = 4.184 J, 1 kg mol = 103 g mol.
RT = (1.984 M) × 300 cal g = 595.2 M cal g
where M = molecular weight of solvent.
O||
Acetone CH3– C–CH ,3 M = 58 g/g mol
O||
,2 M = 72 g/mol
Methylethyl ketone CH3– C–CH –CH
2
Methylethyl ketone:
(
)
595.2
CED = 0.8 g cm 3 106 −
cal
72
= 0.8 [106 − 8.27] cal cm 3
= 78.18 cal cm 3
Acetone:
(
)
595.2
CED = 0.8 g cm 3 125 −
cal
58
= 0.8 [125 − 10.26]
= 91.79 cal cm 3
Since the CED of methylethyl ketone is closer to that of polystyrene than that of acetone, methylethyl
ketone should be a better solvent for polystyrene than acetone.
B. CRYSTALLINE AND AMORPHOUS STRUCTURE OF POLYMERS
As discussed earlier, when a polymer is cooled from the melt or concentrated from a dilute solution,
molecules are attracted to each other forming a solid mass. In doing so, two arrangements are essentially
possible:
• In the first case, the molecules vitrify, with the polymer chains randomly coiled and entangled. The
resulting solid is amorphous and is hard and glassy.
• In the second case, the individual chains are folded and packed in a regular manner characterized by
three-dimensional long-range order. The polymer thus formed is said to be crystalline.
We must recall, however, that polymers are made up of long molecules; therefore, the concept of
crystallinity in polymers must be viewed slightly differently from that in low-molecular-weight substances. Complete parallel alignment is never achieved in polymeric systems. Only certain clusters of
chain segments are aligned to form crystalline domains. These domains, as we shall see shortly, do not
have the regular shapes of normal crystals. They are much smaller in size, contain many more imperfections, and are connected with the disordered amorphous regions by polymer chains that run through
both the ordered and the disordered segments. Consequently, no polymer is 100% crystalline.
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CHEMICAL BONDING AND POLYMER STRUCTURE
1. Crystallization Tendency
Secondary bonding forces, as we saw earlier, are responsible for intermolecular bonding in polymers.
You will recall also that these forces are effective only at very short molecular distances. Therefore, to
maximize the effect of these forces in the process of aggregation of molecules to form a crystalline solid
mass, the molecules must come as close together as possible. The tendency for a polymer to crystallize,
therefore, depends on the magnitude of the inherent intermolecular bonding forces as well as its structural
features. Let us now discuss these in further detail.
Table 3.8 Properties of Polyisoprene Isomers
Isomer
1,4-cis=polyisoprene
(heavea rubber)
Structure
CH3
H
C
C
CH2
1,4-trans-polyisoprene
(gutta-percha)
CH2
CH3
H
C
CH3
Properties
C
Soft, pliable, easily soluble
rubber; has a high retractive
force; used for making
vehicle tires
Tough, hard; used as golf
ball covers
CH2
2. Structural Regularity
We have just said that in the process of association of polymer molecules to form a solid mass, molecules
must come as close together as possible. It follows that any structural features of polymer molecules
that can impede this process will necessarily detract from crystallinity. Polyethylene is perhaps the
simplest molecule to consider in this case. Polyethylene is nonpolar, and the intermolecular attraction
is due to the relatively weak van der Waals forces. The chains can readily assume a planar zigzag
conformation characterized by a sequence of trans bonds and can therefore produce short identity periods
along the polymer chain length. The rotation around the C–C bond is inhibited by an energy barrier of
about 2.7 kcal/mol of bonds. Thus, even though polyethylene molecules are held together by weak van
der Waals forces, the high structural regularity that permits close packing of the chains coupled with
the limited chain flexibility leads to an unexpectedly high melting point (Tm = 135°C), relatively high
rigidity, and low room-temperature solubility. However, as irregularities are introduced into the structure,
as with low-density polyethylene (LDPE), the value of these properties shows a significant reduction.
The crystalline melting point of polyethylene, for example, is reduced 20 to 25°C on going from the
linear to the branched polymer.
Regularity per se is not sufficient to ensure crystallizability in polymers. The spatial regularity and
packing are important. To illustrate this, let us consider two examples of stereoregular polymers. Table 3.8
shows the properties of two isomers: cis- and trans-polyisoprene. It is obvious from the table that the
stereoregular trans form is more readily packed and crystallizable and has properties of crystalline polymers.
The second example is the stereoregularity displayed by monosubstituted vinyl polymers of olefins.
As we saw earlier, these types of polymers can occur in three forms of tacticity: isotactic, syndiotactic,
and atactic. Isotactic and syndiotactic polymers possess stereoregular structures. Generally these polymers are rigid, crystallizable, high melting, and relatively insoluble. On the other hand, atactic polymers
are soft, low melting, easily soluble, and amorphous.
3. Chain Flexibility
In the preceding discussion, we consistently emphasized that close alignment of polymer molecules is
a vital prerequisite for the effective utilization of the intermolecular bonding forces. During crystallization, this alignment and uniform packing of chains are opposed by thermal agitation, which tends to
induce segmental, rotational and vibrational motions. The potential energy barriers hindering this rotation
range from 1 to 5 kcal/mol, the same order of magnitude as molecular cohesion forces. It is to be
expected, therefore, that those polymers whose chains are flexible will be more susceptible to this thermal
agitation than those with rigid or stiff chain structure.
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POLYMER SCIENCE AND TECHNOLOGY
The flexibility of chain molecules arises from rotation around saturated chain bonds. With a chain of
–CH2– units as a basis, it is interesting to consider how variations on this unit will affect rotation of
adjacent units and, hence, chain flexibility. Studies of this type have led to the following general conclusions:
• Rapid conformational change due to ease of rotation around single bonds occurs if such groups as
(– –CO–O–), (–O–CO–O–), and (–C–N–) are introduced into the main chain. If they are regular and/or
if there exist considerable intermolecular forces, the materials are crystallizable, relatively high melting,
rigid, and soluble with difficulty. However, if they occur irregularly along the polymer chain, they are
amorphous, soft, and rubbery materials.
Table 3.9 Effect of Chain Flexibility of Crystalline Melting Point
Polymer
Polyethylene
Repeating Unit
CH2
Tm (°C)
135
CH2
Polyoxyethylene
CH2
Poly(ethylene suberate)
O(CH2)2
Nylon 6,8
NH(CH2)6 NHCO(CH2)6 CO
235
Poly (p-xylene)
CH2
400
CH2
65
O
OCO
(CH2)6 CO
CH2
45
• Ether and imine bonds and double bonds in the cis form reduce the energy barrier for rotation of the
adjacent bonds and “soften” the chain by making polymers less rigid, more rubbery, and more readily
soluble than the corresponding chain of consecutive carbon–carbon atoms. If such “plasticizing” bonds
are irregularly distributed along the polymer chain length, crystallization is inhibited.
• Cyclic structures in the backbone and polar group such as –SO2–, and –CONH– drastically reduce
flexibility and enhance crystallizability.
Table 3.9 illustrates the effect of these factors on crystalline melting point.
4. Polarity
When molecules come together and aggregate into a crystalline solid, a significant cohesion between
neighboring chains is possible. Consequently, polymer molecules with specific groups that are capable
of forming strong intermolecular bonding, particularly if these groups occur regularly without imposing
valence strains on the chains, are crystallizable. You will recall from our earlier discussion that such
groups as
H O
H O H
H O
| ||
| || |
| ||
–N– C– (amide); –N– C–O– (urethane); and –N– C– N– (urea)
(Str. 10)
provide sites for hydrogen bonding whose energy ranges from 5 to 10 kal/mol. In nylon 6 or 6,6, for
example, the regular occurrence of amide linkages leads to a highly crystalline, high melting polymer.
Molecules whose backbone contains –O– units or with polar side groups (–CN, –Cl, –F, or –NO2)
exhibit polar bonding. The bonding energies of such dipoles or polarizable units are in the range between
hydrogen bonding and van der Waals bonding. If these groups occur regularly along the chain (isotactic
and syndiotactic), the resulting polymers are usually crystalline and have higher melting points than
polyethylene (Table 3.10).
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CHEMICAL BONDING AND POLYMER STRUCTURE
Table 3.10 Effect of Polarity on Crystallizability
Polymer
Polyethylene
Nylon 6
Repeat Unit
CH2
Tem (°C)
135
CH2
H
O
N
C
223
(CH2)5
H
H
O
N
C
O
Nylon 6,6
N
Polyoxymethylene
CH2
O
180
Poly(vinyl chloride)
CH2
CH
273
(CH2)6
(CH2)4
C
265
Cl
Polyacrylonitrile
CH2
CH
317
CN
Given our earlier argument that the presence of –O– units in a chain backbone enhances flexibility,
the fact that the melting point of polyoxymethylene (180°C) is higher than that of polyethylene (135°C)
(Table 3.10) seems contradictory. However, the dipole character of the C–O–C group produces polar
forces between adjacent chains that act over a longer range and are stronger than van der Waals forces.
Thus, for polyoxymethylene the induced flexibility is more than offset by the increased bonding forces
resulting from polarity.
5. Bulky Substituents
The vibrational and rotational mobility of intrinsically flexible chains can be inhibited by bulky substituents; the degree of stiffening depends on the size, shape, and mutual interaction of the substituents.
For example, vinyl polymers with small substituents such as polypropylene [–CH3] and polystyrene
[–C6H5] can crystallize if these pendant groups are spaced regularly on the polymer chain as in their
isotactic and syndiotactic forms. In their atactic forms, the randomly disposed pendant groups prevent
the close packing of the chains into crystalline lattice. The atactic forms of these polymers are therefore
amorphous. Large or bulky substituents, on the other hand, increase the average distance between chains
and, as such, prevent the effective and favorable utilization of the intermolecular bonding forces. Thus
O
||
polymers like poly(methyl acrylate) and poly(vinyl acetate) with large pendant groups – C–O–CH 3 and
O
||
O– C–CH 3, respectively, cannot crystallize even if the pendant groups are spaced regularly (isotactic and
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POLYMER SCIENCE AND TECHNOLOGY
syndiotactic forms). Table 3.11 shows the change in the crystalline melting point with increased length
of the polymer side chain. Note that for the polyolefins, polyethylene, polypropylene, poly(1-butene),
and poly(1-pentene), the melting point shows a maximum for polypropylene. A large pendant group in
close proximity of the main chain stiffens the chain. However, when the size of the pendant group is
such that the packing distance between the chains in the solid state is increased, the forces of interaction
between chains decrease and so does the melting point. The presence of an aromatic side group in
polystyrene considerably stiffens the chain, which has a stable helix form in the solid state. The helices
pack efficiently to allow greater interchain interaction.
The above discussion clearly indicates that stereo regularity, chain flexibility, polarity, and other steric
factors have profound influence on crystallizability and melting points and, hence, as we shall see later,
play an important role in the thermal and mechanical behavior of polymers.
Table 3.11 Stiffening of Polymer Chains
by Substituents
Polymer
Repeat Unit
Tm (°C)
Polyethylene
CH2
CH2
135
Polypropylene
CH2
CH
176
CH3
Poly(1-butene)
CH2
CH
125
CH2
CH3
Poly(1-pentene)
CH2
CH
75
CH2
CH2
CH3
Polystyrene
CH2
CH
240
Example 3.5: Explain the following observations.
a. Atactic polystyrene can be oriented (have its chain aligned by stretching at a temperature above
its Tg, but does not crystallize; rubber on the other hand, both crystallizes and becomes oriented
when it is stretched.
b. Poly(vinyl alcohol) is made by the hydrolysis of poly(vinyl acetate) because vinyl alcohol
b.
monomer is unstable. The extent of reaction may be controlled to yield polymers with anywhere
from 0 to 100% of the original acetate groups hydrolyzed. At room temperature, pure poly(vinyl)
acetate), i.e., 0% hydrolysis, is insoluble in water. However, as the extent of hydrolysis is
increased, the polymers become more water soluble up to 87% hydrolysis, after which further
hydrolysis decreases water solubility.
c. Toluene and xylene have approximately the same cohesive energy density (CED), but xylene is
a more convenient solvent for polyethylene.
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CHEMICAL BONDING AND POLYMER STRUCTURE
Solution:
a. Atactic polystyrene has an irregular structure and therefore will not crystallize when oriented. Natural
rubber (cis-1,4-polyisoprene) has a relatively small pendant group and as such is crystallizable.
O
CH2
CH
+
O
C
H2O
CH2
CH
+ CH3
C
OH
OH
O
CH3
poly(vinyl acetate)
poly(vinyl alcohol)
acetic acid
(Str. 11)
As the hydroxyl group becomes available following the hydrolysis, there is a corresponding
increase in the water solubility of the product, poly(vinyl alcohol), as a result of intermolecular
hydrogen bond formation. At an advanced stage of hydrolysis the CED of poly(vinyl alcohol)
increases to such an extent that water solubility decreases, from the arguments in Section V.A.
c. Toluene has a boiling point of about 111°C, while xylene has a boiling point of 138 to 144°C.
As a result of the highly crystalline nature of polyethylene it will dissolve in solvents only at
temperatures close to its melting point (about 135°C). When the boiling points of toluene and
xylene are compared, xylene is obviously a preferable solvent for polyethylene.
C. MORPHOLOGY OF CRYSTALLINE POLYMERS
Most polymers are partially crystalline. Evidence for this emerged in the1920s from X-ray diffraction
studies. X-ray diffraction patterns of some polymers, in contrast to those of simple crystalline solids,
showed sharp features, associated with regions of three-dimensional order, superimposed on a diffuse
background characteristic of amorphous, liquidlike substances. The interpretation of these patterns was
that polymers are semicrystalline, consisting of small, relatively ordered regions — the crystallites
embedded in an otherwise amorphous matrix. This interpretation led to the “fringed micelle” model of
crystalline polymers. The fringed micelle concept, which enjoyed popularity for many years, held that,
since polymer chains are very long, they passed successively through the crystallites and amorphous
regions (Figure 3.10). The chains were thought to run parallel to the longer direction of the crystallites.
Although the fringed micelle model of polymer morphology seemed to explain many of the properties
of semicrystalline polymers, it has now been abandoned in favor of more ordered and complex models.
This change is partly as a result of developments in the field of electron microscopy.
The morphology of crystalline polymers — that is, the size, shape, and relative magnitude of
crystallites — is rather complex and depends on growth conditions such as solvent media, temperature,
and growth rate. In discussing polymer crystalline morphology, our initial focus is on molecular packing.
This concerns how the polymer chains (with an extended conformation of either planar zigzag or helix)
are packed into the unit cell, which is the fundamental element of a crystal structure. This is followed
by discussion of the morphologic features of the polymer single crystal and those of polymers crystallized
from the melt.
1. Crystal Structure of Polymers
The fully extended planar zigzag (trans conformation) is the minimum energy conformation for an
isolated section of polyethylene or paraffin hydrocarbon. The energy of the trans conformation is about
800 cal/mol less than that of the gauche form. Consequently, the trans form is favored in polymer crystal
structures. Typical polymers that exhibit this trans form include polyethylene, poly(vinyl alcohol),
syndiotactic forms of poly(vinyl chloride) and poly(1,2-butadiene), most polyamides, and cellulose. Note
that trans conformation is different from the trans configuration discussed in Section IV.A.
In some cases, however, steric hindrance causes the main chain to assume a minimum energy
conformation other than the trans form. Some of these variations may be mere distortions of the fully
extended planar zigzag conformation, as in most polyesters, polyisoprenes, and polychloroprene. In other
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POLYMER SCIENCE AND TECHNOLOGY
Figure 3.10 Fringed micelle model. (From Bryant, W.W.D., J. Polym. Sci., 2, 547, 1947. With permission.)
cases, in order to relieve the strain due to the presence of bulky substituents, the main chain rotates and
assumes the helical conformation in the crystalline phase. This is the case for most isotactic polymers
and 1,1 disubstituted ethylenes.
a. Polyethylene
The unit cell in polyethylene is a parallelepiped with a rectangular cross-section and lattice parameters:
a = 7.41 Å; b = 4.94 Å; and c = 2.55 Å (orthorhombic crystal system) (Figure 3.11). By convention,
the polymer chains, in passing through the unit cell, lie parallel to the lattice translation vector c. The
lattice parameter c (magnitude of c) depends on the crystallographic repeat unit (Bravais lattice or crystal
system). In polyethylene, the crystallographic repeat unit contains one chemical repeat unit. The packing
of the repeat units in the unit cell is shown in Figure 3.12.
Figure 3.11 Arrangement of chains in the unit cell of polyethylene. (From Geil, P.H., Polymer Single Crystals,
John Wiley & Sons, New York, 1933. With permission.)
b. Poly(ethylene terephthalate)
The conformation is a slight distortion of the planar zigzag. The benzene ring lies nearly in the plane
of zigzag, but the main chains are no longer exactly planar. They make a slight angle with the planar
zigzag (Figure 3.13). The unit cell has lattice constants:
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CHEMICAL BONDING AND POLYMER STRUCTURE
83
Figure 3.12 Packing in the crystal structure of polyethylene as viewed along the c-axis. (From Natta, G. and
Corradini, P., Rubber Chem. Technol., 33, 703, 1960. With permission.)
Figure 3.13 Molecular conformation of polyethylene terephthalate.
a = 4.56 Å
b = 5.94 Å
c = 10.75 Å
α = 98.5°
β = 118°
δ = 112°
c. Polypropylene
We have first seen that polyethylene exists in the planar zigzag conformation. Polypropylene can be
considered as having a linear polyethylene backbone, but with the H atom on every other carbon atom
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POLYMER SCIENCE AND TECHNOLOGY
replaced by a methyl [–CH3] group. Polypropylene can exist in either atactic (noncrystallizable) form
or in the crystallizable syndiotactic or isotactic forms. For the isotactic form, because of the size of the
pendant [–CH3] group (relative to the H atom, in polyethylene), the backbone can no longer exist in the
planar zigzag form; it must rotate. The lowest energy state is attained by a regular rotation of 120° by
each chemical repeat unit. This means that there are three chemical repeat units per turn. These pack
into a monoclinic crystal system (Figure 3.14) whose unit cell has parameters:
Figure 3.14 Projection of the monoclinic unit cell of polypropylene along the chain-axis. (From Natta, G. and
Corradini, P., Nuovo Cimento Suppl., 15(1), 40, 1960. With permission.)
a = 6.65 Å + 0.05 Å
b = 20.96 + 0.15 Å
c = 6.50 + 0.04 Å
β = 99.° 20′ + 1°
d. Degree of Crystallinity
We noted earlier that polymers, by virtue of their large size and in contrast to low-molecular-weight
materials, are incapable of 100% crystallinity. To visualize this mentally, the term semicrystalline is
frequently used to describe crystalline polymers. One of the most useful and practical concepts in the
characterization of semicrystalline polymers is the degree of crystallinity. Let us now consider how this
can be estimated.
We start by treating the semicrystalline polymer as a two-phase system with a distinct demarcation
between the crystalline and amorphous material. We know, of course, that this is not strictly true. Now
suppose Pm is an actual or measured intensive property of the polymer, while Pc and Pa are the same
property due, respectively, to the crystalline and amorphous materials (components) in the same state
as exists in the polymer. Then the degree of crystallinity can be deduced from the individual contributions
of the crystalline and amorphous components to the measured property:
p m = φ p c + (1 − φ) pa
By rearrangement, Equation 3.12 becomes
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(3.12)
CHEMICAL BONDING AND POLYMER STRUCTURE
φ=
pa − p m
pa − pc
85
(3.13)
The degree of crystallinity may be derived this way by measurement of a material property such as
specific volume, specific heat, enthalpy, and electrical resistivity:
Example 3.6: For the polyethylene of Figure 3.15 (lower curve) calculate the fraction of crystalline
material at 20°C assuming the coefficient of expansion for amorphous material is the same above and
below Tm .
Figure 3.15 Specific volume–temperature relations for linear polyethylene (Marlex 50). Specimen slowly cooled
from melt to room temperature prior to experiments (0) and specimen crystallized at 130°C for 40 days and then
cooled to room temperature prior to experiment (0). (From Mardelkern, L., Rubber Chem. Technol., 32, 1392,
1959. With permission.)
Solution:
φ=
Va − Vm
Va − Vc
where Va = specific volume due to the amorphous component
Vm = measured specific volume
Vc = specific volume due to the crystalline component
At 130°C, the crystallization temperature, the specific volume would be due to the amorphous phase.
From Figure 3.15, Va = 1.100 and Vm = 1.06 (at 20°C). To calculate the specific volume of the ideal
polymer remember that
Density of a perfect crystal = ρ where
ρ=
Mass
nM
=
Volume N A V
where n = number of polymer repeat units per unit cell
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POLYMER SCIENCE AND TECHNOLOGY
M = molecular weight of repeat unit unit
NA = Avogadro’s number
V = volume of a unit cell
For polyethylene, the unit cell is orthorhombic with lattice parameters:
a = 7.41 Å
b = 4.94 Å
c = 2.55 Å
Besides, each unit cell contains two repeat units, one at the center, four at the corners, but with each
shared with four other unit cells. That is a total contribution of one repeat unit by the corners (see
Figure 3.12).
ρ=
2 × 28
(6.023 × 10 ) (7.41 × 10 ) (4.94 × 10 ) (2.55 × 10 )
23
−8
−8
−8
= 0.996 g cm 2
VC = specific volume = 1 ρ = 1.004
φ=
1.100 − 1.016 0.084
=
= 0.875
1.100 − 1.004 0.096
2. Morphology of Polymer Single Crystals Grown from Solution
For a long time, it was believed that, because of the molecular entanglements of polymer chains in
solution, it would be impossible to produce polymer single crystals. The first report of the growth of
polymer single crystals from dilute solution was in 1953. This was followed by several other reports
and for so many polymers this phenomenon is now regarded as universal. Growth of polymer single
crystals requires crystallization from dilute solutions at relatively high temperatures by cooling from a
temperature above the crystalline melting point. Different morphologies result depending on polymer
type and growth conditions.
a. Lamellae
All polymer single crystals have the same general appearance. Under an electron microscope, they appear
as thin, flat platelets that are 100 to 120 Å thick and several microns in lateral dimensions. This lamellar
nature of polymer single crystals has been found to be fundamental. Growth of the crystal normal to
lamellar surface occurs by the formation of additional lamellae of the same thickness as the basal
lamellae; thick crystals are usually multilamellar.
b. Chain Folding
Figure 3.16 is an electron micrograph of crystals of polyethylene obtained by cooling a dilute solution
(0.1%, in tetrachloroethylene). Such electron microscopy and diffraction studies have confirmed not only
the lamellar nature of single crystals but have also revealed that the polymer molecules are oriented
normal (or, in some cases, very nearly normal) to the lamellar surface.
Since polymer molecules are generally 1000 to 10,000 Å long and lamellae are only 100 Å thick, it
follows that chains must fold repeatedly on themselves. For polyethylene, for example, it has been
demonstrated that only about five chain carbon atoms are required for the chain to fold on itself.
The plane on which the regular folding of chains occurs defines the fold plane, while the thickness
of the lamella is regarded as the fold period (Figure 3.16). Chain ends may either terminate within the
crystal, forming a defect, or be excluded from the crystal, forming cilia. In some cases, irregular folding
and branch points can also occur.
Let us now consider how this fold conformation fits into the overall morphological features of the
polymer single crystal. Figure 3.17 is a schematic representation of the top surface of an idealized model
of a diamond-shaped polyethylene single crystal as seen along the (001) (c axis). The curved lines,
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CHEMICAL BONDING AND POLYMER STRUCTURE
87
Figure 3.16 Schematic diagram of chain folding showing conformational imperfections.
Figure 3.17 Fold packing in a polyethylene single crystal. (From Reneker, D.H. and Geil, P.H., J. Appl. Phys.,
31, 1916, 1960. With permission.)
which represent the chain folds, terminate on solid lines representing the plane of zigzag. Note the
following features:
• The alternating parallel orientation of the planes of zigzag indicates that the chains twist in addition
to folding within their fold plane.
• The crystal is divided into four quadrants (broken lines) and, as we shall see shortly, each sector slopes
away from the apex of the pyramidal structure. The fold planes in each quadrant are parallel to the
outside edge of that quadrant. It follows that the entire crystal is composed of four triangular quadrants
that contain rows of fold planes.
c. Hollow Pyramidal Structure
Ridges are formed by pleats of extra material deposited along one of the diagonals of each diamondshaped crystal. Obviously, therefore, crystals of polyethylene are not simply flat lamellae. Experimental
evidence has shown that they may exist in solution as hollow pyramids. These pyramids may or may
not be corrugated depending on the crystallization conditions.
The hollow pyramidal structure is due to the packing of the folded chains in which successive planes
of folded molecules are displaced from their neighbors by an integral of repeat distances. In some cases,
the fold and fold period are regular, and the displacement of adjacent fold planes is uniform. This results
in the formation of a planar pyramid. In other cases, however, the direction of displacement is reversed
periodically. In this case, corrugated pyramids are formed.
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POLYMER SCIENCE AND TECHNOLOGY
Besides the structures already discussed, more complex morphologies may be obtained from the
growth of polymer crystals from solutions. The structure that emerges from the crystallization of a
polymer is a function of a complex interaction of factors that include the type of solvent, solution
temperature, concentration, and polymer molecular weight. Some examples of these structures include
spiral growth, dendrites, and hedrites.
Figure 3.18 The intercrystalline fibril.
3. Morphology of Polymers Crystallized from the Melt
The most prominent structural feature of polymers crystallized from the melt is the spherulite. The
spherulite is not a single crystal, but an extremely complex spherical aggregate of lamellae ranging in
size from about 0.1 µ to possibly a few millimeters in diameter. Under a polarizing microscope,
spherulites show characteristic dark Maltese cross patterns arising from the birefringent effects associated
with the molecular orientation of lamellae morphology. When a sample of a crystallizable polymer like
polyethylene, nylon 6,6, or poly(ethylene terephthalate) is heated above its melting temperature and then
supercooled by about 10 to 15°C, spherulite growth is indicated at several centers. In the case of
homogenous nucleation these centers arise spontaneously in the melt, while for heterogeneous nucleation
the nucleation center is a foreign body. During growth, spherulites expand radially at a constant linear
rate until the growth fronts from neighboring spherulites impinge.
Electron microscopy and electron diffraction studies have revealed that for almost all polymers, spherulites
are composed of lamellar structure. Each lamella is a flat ribbon, and, like in simple crystals, chains that are
folded are oriented perpendicular to the surface of the lamella. The growth nucleus for crystallization or
spherulite development is thought to be a simple crystal that develops by the formation of a multilayer stack.
Thereafter, one axis of each lamella extends forming a lamellar fibril. These lamellar fibrils now grow radially
from a central nucleus, but have a tendency to twist, diverge, and branch during growth. Since individual
lamella do not increase in lateral dimensions, their characteristic branching via screw dislocations is a spacefilling process. Lamellae usually have dimensions of 1 µ in length and 100 Å in thickness.
As crystallization proceeds, the growth fronts of two different spherulites meet, and the lamellae extend
across spherulite boundaries into uncrystallized material available, thus holding the material together. In
addition, interlamellar fibrils tie two or more lamellae together and also bridge spherulite themselves.
One important feature of the interlamellar fibril is that the chain molecules lie parallel to the length
of the fibril in contrast to the situation existing in the lamella where chains are oriented at right angles
(Figure 3.18). This implies that each interlamellar fibril is an extended chain crystal. The formation of
interlamellar links is thought to originate from the inclusion of one chain molecule in two different and
possibly widely separated lamellae. This provides the nucleus for subsequent deposition of other molecules, thus producing the intercrystalline ties.
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CHEMICAL BONDING AND POLYMER STRUCTURE
89
The picture that emerges from the above discussion of the development of spherulitic structure is that
spherulites represent the crystalline portion of a sample growing at the expense of the noncrystallizable
material. The amorphous regions therefore constitute the residual elements of disorder resulting from the
fact that the noncrystallizable material in the original melt — which includes catalyst residues, nonstereoregular chains (e.g., atactic chain segments), short-chain components, plasticizer molecules, and chain
ends (low-molecular-weight chains) — is unable to disentangle and rearrange itself into the ordered arrays
required in the crystalline state. It appears that as spherulite growth proceeds the noncrystallizable material
diffuses ahead of the growth front. However, spherulites themselves, while predominantly crystalline, do
contain defects such as chain ends, dislocations, and chemical impurities. The defect materials segregate
and separate the radiating lamellae and contribute to the overall amorphous content. Consequently,
polymers have a wide variety of crystallinity, which varies from 0% in noncrystallizable polymers like
atactic PMMA to almost 100% for highly crystallizable polymers like polytetrafluoroethylene and linear
polyethylene. For a particular crystallizable polymer, the degree of crystallinity depends on spherulite
growth conditions, which determine the size and extent of perfection of the crystals. For example, it is
known that higher degrees of lamellar perfection can be obtained generally at high crystallization temperatures (in the neighborhood of the Tm polymers and after prolonged periods at these temperatures.
Given the long-chain nature of polymer molecules, it is obvious that the process of crystallization
involves extensive molecular translation from the high degree of disorder characteristic of the melt to
the highly ordered state. Also, this must occur in a time that is short relative to the time required for
crystallization. Consequently, the degree of crystallinity in most polymers is a function of the rate of
crystallization. After rapid crystallization, the amorphous content of the polymer sample is increased.
On the other hand, if a molten polymer is crystallized slowly, the crystals develop in a more perfect
manner and tend to exclude impurities that could interfere with the ordering process.
VI.
CRYSTALLINITY AND POLYMER PROPERTIES
To conclude this discussion, we need to examine how crystallinity is related to polymer properties. As
we have seen above, polymers are semicrystalline, which means that they are composed of amorphous
and crystalline phases. Since the amorphous phase can exist in the rubbery or glassy state, the overall
effect of the semicrystalline nature of crystalline polymers depends, in the first place, on the state of the
amorphous phase or the temperature of use. For example, the modulus of crystalline polymers is only
about an order of magnitude higher than the modulus of an amorphous polymer in the glassy state,
whereas it is about four orders of magnitude higher than the modulus of the amorphous in the rubbery
state. This suggests therefore that modification of polymer properties due to crystallization will be more
pronounced for a polymer with amorphous component in the rubbery state than for one whose amorphous
phase is in the glassy state. For example, the modulus of rubber can be increased dramatically by induced
crystallization. However, for polystyrene — whose amorphous component is glassy at room
temperature — crystallization, if induced, has a negligible effect on its modulus, which is already high.
By similar arguments, it can be seen that any polymer property that is different for both the amorphous
and crystalline components of the polymer will be determined by the relative amounts of these two
components as well as their form and distribution (i.e., by the polymer morphology). It is therefore
obvious that for engineering design of polymers, control of the properties of a semicrystalline polymer
resolves into control of its morphology or spherulite development process. The size and degree of
perfection of spherulites are controlled by the crystallization conditions that exist during the unit
operations involved in the production of a polymer product.
Example 3.7: Poly(ethylene terephthalate) is cooled rapidly from 300°C (state 1) to room temperature
(state 2). The resulting material is rigid and perfectly transparent. The sample is then heated to 100°C
and maintained at that temperature during which time it gradually becomes translucent (state 3). It is
then cooled down to room temperature and is again found to be rigid, but is now translucent rather than
transparent (state 4). For this polymer, Tm = 267°C and Tg = 69°C. Describe the molecular mechanisms
responsible for the behavior of the polymer in each state. Sketch a general specific-volume vs. temperature
curve for this polymer indicating Tg, Tm, and the locations of states 1–4 described above.
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POLYMER SCIENCE AND TECHNOLOGY
Solution:
Crystallinity is important in determining optical properties because the refractive index of the crystalline
region is always higher than that of the amorphous component irrespective of whether the amorphous
component is in the glassy or rubbery state. This difference in refractive indices of the component phases
leads to high scattering and consequently, the translucency or haziness of semicrystalline polymers. For
a purely amorphous polymer, this does not occur, and hence amorphous polymers are usually transparent.
Therefore the state of polyethylene terephthalate can be explained as follows:
State
Polymer Properties
1
2
Polymer is in the melt form and therefore completely amorphous.
As a result of rapid cooling, polymer molecules are unable to align themselves for crystallization. Polymer is
therefore amorphous, glassy, and transparent.
At 100°C which is higher than the Tg of the polymer, some molecular (segmental) mobility of polymer is now
possible. Given sufficient time at this temperature, molecular alignment occurs for crystallization to take place.
Polymer is now semicrystalline since crystallization cannot be 100%. The differing refractive indices of the
amorphous and crystalline components result in translucency.
When cooled to this state, polymer retains its semicrystalline nature and hence its translucency.
3
4
VII.
PROBLEMS
3.1 Explain the following observations:
a. Polyisobutylene has a much higher oxidation resistance than natural rubber.
b. Polyethylene is a better material for transformer insulation than poly(p-chlorostyrene).
c. At room temperature, poly(methylmethacrylate) is transparent, while polyethylene is translucent. At
temperatures above 135°C, both polymers are transparent.
d. Polyethylene assumes a planar zigzag conformation, while polyisobutylene has a helical conformation.
e. The percentage of moisture absorption of the following polyamides:
Nylon 6: 1.3–1.9
Nylon 12: 0.25–0.3
f. Small amounts of chlorination (10 to 50 wt%) of polymethylene (–CH2–), a saturated paraffin, lower
its softening temperature. However, with higher amounts of chlorination (>70 wt%) the softening
temperature of the polymer increases again.
g. The addition of glass fibers increases the heat distortion temperature of crystalline polymers like polyethylene and polypropylene, but not that of amorphous polymers like polycarbonate and polysulfone.
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CHEMICAL BONDING AND POLYMER STRUCTURE
h. Both polyethylene and gasoline are hydrocarbons, but high-density polyethylene (HDPE) is used for
making automobile gasoline tanks.
i. Teflon is used in nonstick pans.
j. Nylon 6 has a higher melting point than polyethylene.
3.2. A sample of styrene monomer is heated until it is partially polymerized. In order to determine the quantity
of polymer formed, the solution of polystyrene in unreacted styrene monomer is poured into a large
volume of methanol. The polymer precipitates (methanol has a considerably higher cohesive energy density
than polystyrene) and is readily dried and weighed. The CED of water is even higher than that of methanol.
Why is water not used in place of methanol for precipitating the polymer? Could the quantity of polymer
be determined by distilling off the unreacted monomer and weighing the residue? Explain.
3.3. Two hundred grams of polymer consist of the fractions shown in the following table:
Fraction
Mass
(g)
Molecular Weight
(g/gmol)
I
II
III
100
50
50
2 × 103
2 × 104
1 × 105
What are the values of Mn, Mw , and the polydispersity of the sample.
3.4. Three samples of a polymer were mixed thoroughly without reaction as shown below. Calculate Mn and Mw .
Sample
Mn
Mw
Mass in
Mixture (g)
A
B
C
1.2 × 105
5.6 × 105
10.0 × 105
4.5 × 105
8.9 × 105
10.0 × 105
200
200
100
3.5. The number-average degrees of polymerization required for good mechanical properties by polymers V
and P are 2000 and 1500, respectively. It is known, however, that one of these polymers is poly(vinyl
chloride), while the other is poly(vinylidene chloride). On the basis of this information assign polymers V
and P. Explain your decision.
3.6. In most cases, free-radical polymerization of a monomer containing a C› C double bond results in a
noncrystalline polymer. Explain. Give three examples of monomers that yield crystalline polymers by
free-radical polymerization.
3.7. Bristow and Watson1 reported the following data at 25°C for a 1:39 copolymer of acrylonitrile and butadiene.
Solvent
2,2,4-trimethylpentane
n-Hexane
CCl4
CHC;3
Dioxane
CH2Cl2
CHBr3
Acetonitrile
∆Hvap
(cal/mol)
V1
(cm3/mol)
V2 (vol fraction)
(polymer in gel)
8,396
7,540
7,770
7,510
8,715
7,004
10,385
7,976
166.0
131.6
97.1
80.7
85.7
64.5
87.9
52.9
0.9925
0.9737
0.6862
0.1510
0.2710
0.1563
0.1781
0.4219
From Bristow and Watson, Trans. Faraday Soc., 54, 1731, 1958. With
permission.
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POLYMER SCIENCE AND TECHNOLOGY
Calculate the cohesive energy density (CED) and δ2 for each solvent. Plot (V2)–1 vs. δ1 to determine δ2
and CED for the polymer. If the heat of mixing is given by
∆H m − V2 (1 − V2 ) (δ1 − δ 2 ) ,
2
calculate the heat of mixing equal volumes of uncross-linked polymer and acetonitrile.
3.8. Explain the following:
a. Polyethylene and polypropylene produced with stereo-specific catalysts are each fairly rigid, translucent plastics, while a 65:35 copolymer of the two produced in exactly the same manner is a soft,
transparent rubber.
b. A plastic is commercially available that is similar in appearance and mechanical properties to polyethylene and polypropylene in (a) but consists of 65% ethylene and 35% propylene units. The two
components of this plastic cannot be separated by any physical or chemical means without degrading
the polymer.
3.9. The polymers of amino acids are termed nylon n where n is the number of consecutive carbon atoms in
the chain. Their general formula is
H
O
N
C
(Str. 12)
(CH2)n-1
x
The polymers are crystalline and will not dissolve in either water or hexane at room temperature. They
will, however, reach an equilibrium level of absorption when immersed in each liquid. Describe how
and why water and hexane absorption will vary with n.
3.10. Explain why a styrene–butadiene copolymer with solubility parameter δ = 8.1 is insoluble in both
pentane (δ = 7.1) and ethylene acetate (δ = 9.1), but will dissolve in a 1:1 mixture of the two solvents.
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CHEMICAL BONDING AND POLYMER STRUCTURE
3.11. The urea derivatives of the following amines were made by refluxing the amines with stoichiometric
amounts of urea. Explain the observed crystallization tendencies and melting points of the resulting
urea derivatives of amines.
Amine
Crystallization Tendency
Crystals formed on cooling the reflux product;
mp 200°C
H2N (CH2)6 NH2
Hexamethylenediamine
H2N
CH2
N
CH2
CH2
Crystals formed on cooling and leaving the
reflux product overnight; mp = 165°C
NH2
CH2
CH2
NH2
Tris(2-aminoethylamine)
H2N CH2CH2O CH2 CH2O
Triethyleneoxide-diamine
CH2
CH3CH2
C
OCH2 CH(CH3)
CH2 O
CH2
CH2 CH2 NH2
CH2CH(CH3)
x
OCH2 CH(CH3)
NH2
NH2
y
z
Crystals formed on cooling and leaving the
reflux product for 3 weeks; mp = 135°C
Viscous liquid
NH2
Poly(oxypropylene) triamine x + y + z = 5.3
3.12. For a crystallizable polymer the degree of crystal imperfection is much higher in bulk-crystallized
material than in material crystallized from dilute solution. However, the level of crystal perfection can
be significantly enhanced if the polymer melt is left for sufficient time at relatively high temperature.
REFERENCES
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17.
Gaylord, N.G. and Mark, H., Linear and Stereoregular Addition Polymers, Interscience, New York, 1959.
Van Krevelen, D.W., Properties of Polymers, Elsevier, Amsterdam, 1972.
Bryant, W.W.D., J. Polym. Sci., 2, 547, 1947.
Geil, P.H., Polymer Single Crystals, John Wiley & Sons, New York, 1933.
Natta, G. and Corradini, P., Rubber Chem. Technol., 33, 703, 1960.
Daubency, R. dep, Brian, C.W., and Brown, J.C., Proc. R. Soc. London, 226A, 531, 1954.
Natta, G. and Corradini, P., Nuovo Cimento Suppl., 15(1), 40, 1960.
Reneker, D.H. and Geil, P.H., J. Appl. Phys., 31, 1916, 1960.
Ranby, B.G., Morehead, F.F., and Walter, N.M., J. Polym. Sci., 44, 349, 1968.
Geil, P.H. and Reneker, D.H., J. Polym. Sci., 51, 569, 1975.
Maxwell, B., Modifying polymer properties mechanically, in Polymer Processing, Fear, J.V.D., Ed., Chemical
Engineering Process Symp., Ser. 60(49), 10, 1964.
Keith, H.D., Padden, F.J., and Vadinsky, R.G., J. Polym. Sci., 4(A-2), 267, 1966.
Bristow and Watson, Trans. Faraday Soc., 54, 1731, 1958.
Mardelkern, L., Rubber Chem. Technol., 32, 1392, 1959.
Billmeyer, F.W., Jr., Textbook of Polymer Science, 2nd ed., John Wiley & Sons, New York, 1971.
Kaufman, H.S. and Falcetta, J.J., Eds., Introduction to Polymer Science and Technology, John Wiley & Sons, New
York, 1977.
Williams, D.J., Polymer Science and Engineering, Prentice-Hall, Englewood Cliffs, NJ, 1971.
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POLYMER SCIENCE AND TECHNOLOGY
18. Sharples, A., Introduction to Polymer Crystallization, St. Martin’s Press, New York, 1966.
19. Pauling, L., The Nature of the Chemical Bond, Cornell University Press, Ithaca, NY, 1960.
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Chapter 4
Thermal Transitions in Polymers
I.
INTRODUCTION
When a block of ice is heated, its temperature increases until at a certain temperature (depending on
the pressure) it starts to melt. No further increase in temperature will be observed until all the ice has
melted (solid becomes liquid). If heating is continued, the same phenomenon is observed as before and
as the liquid starts to boil (liquid turns to vapor). It is pertinent to make two observations here:
• Water exists in three distinct physical states — solid, liquid, and gas (vapor).
• Transitions between these states occur sharply at constant, well-defined temperatures.
The thermal behavior of all simple compounds, such as ethanol or toluene, is analogous to that of water.
However, the transitions in polymers are somewhat different and certainly more complex. In the first
place, those molecules large enough to be appropriately termed polymers do not exist in the gaseous
state. At high temperatures, they decompose rather than boil since what we would consider conventionally
as their “boiling points” are generally higher than their decomposition temperatures. Second, a given
polymeric sample is composed of a mixture of molecules having different chain lengths (molecular
weights). In contrast to simple molecules, therefore, the transition between the solid and liquid forms
of a polymer is rather diffuse and occurs over a temperature range whose magnitude (of the order of
2 to 10°C) depends on the polydispersity of the polymer (Figure 4.1). On melting, polymers become
very viscous (viscoelastic) fluids, not freely flowing as in the case of low-molecular-weight materials.
In addition, there is a still more fundamental difference between the thermal behavior of polymers
and simple molecules. To understand, first recall that molecular motion in a polymer sample is promoted
by its thermal energy. It is opposed by the cohesive forces between structural segments (groups of atoms)
along the chain and between neighboring chains. These cohesive forces and, consequently, thermal
transitions in polymers depend on the structure of the polymer. In this regard, two important temperatures
at which certain physical properties of polymers undergo drastic changes have been identified:
• The glass transition temperature, Tg
• The crystalline melting point, Tm
If a polymer is amorphous, the solid-to-liquid transition occurs very gradually, going through an intermediate “rubbery” state without a phase transformation. The transition from the hard and brittle glass
into a softer, rubbery state occurs over a narrow temperature range referred to as the glass transition
temperature. In the case of a partially crystalline polymer, the above transformation occurs only in the
amorphous regions. The crystalline zones remain unchanged and act as reinforcing elements thus making
the sample hard and tough. If heating is continued, a temperature is reached at which the crystalline
zones begin to melt. The equilibrium crystalline melting point, Tm, for polymers corresponds to the
temperature at which the last crystallite starts melting. Again, in contrast to simple materials, the value
of Tm depends on the degree of crystallinity and size distribution of crystallites. The general changes in
physical state due to changes in temperature and molecular weight are shown in Figure 4.2 for amorphous
and crystalline polymers.
The thermal behavior of polymers is of considerable technological importance. Knowledge of thermal
transitions is important in the selection of proper processing and fabrication conditions, the characterization of the physical and mechanical properties of a material, and hence the determination of appropriate
end uses. For example, the glass transition temperature of rubber determines the lower limit of the use
of rubber and the upper limit of the use of an amorphous thermoplastic. We take up discussion of these
transition temperatures in succeeding sections.
0-8493-????-?/97/$0.00+$.50
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POLYMER SCIENCE AND TECHNOLOGY
Figure 4.1 Relative thermal responses of simple molecules (a) and polymers (b).
Figure 4.2 Temperature–molecular weight diagram. (a) For amorphous polymer; (b) for crystalline polymer.
II.
THE GLASS TRANSITION
To illustrate the concept of glass transition, let us consider the specific volume–temperature behavior
for both amorphous (ABCD) and crystalline (ABEF) polymers, as shown in Figure 4.3. As the amorphous
polymer (line ABCD) is heated from the low-temperature region (region D), the volume expands at a
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THERMAL TRANSITIONS IN POLYMERS
97
Figure 4.3 Specific volume–temperature curves for a semicrystalline polymer. (A) Liquid region; (B) viscous
liquid with some elastic response; (C) rubbery region; (D) glassy region; (E) crystallites in a rubbery matrix;
(F) crystallites in a glassy matrix.
constant rate. At a characteristic temperature, Tg, the rate of volume expansion increases suddenly to a
higher constant level, i.e., there is a change in the slope of the volume–temperature curve from a lower
to a higher volume coefficient of expansion. At the same time, there is an abrupt change in physical
behavior from a hard, brittle, glassy solid below Tg (region D) to a soft, rubbery material above Tg
(region C). On further heating, the polymer changes gradually from the rubbery state to a viscous liquid
(region B) whose viscosity decreases with increasing temperature until decomposition sets in.
For a crystalline polymer, the changes at Tg are less drastic. This is because these changes are restricted
mainly to the amorphous domains while the crystalline zones remain relatively unaffected. Between the
glass transition (Tg) and the melting temperature (Tm) (region E) the semicrystalline polymer is composed
of rigid crystallites immersed (dispersed) in a rubbery amorphous matrix. In terms of mechanical
behavior, the polymer remains rigid, pliable, and tough. At the melting temperature, the crystallites melt,
leading to a viscous state (region B). Above Tm the crystalline polymer, like the amorphous polymer,
exists as a viscous liquid.
A. MOLECULAR MOTION AND GLASS TRANSITION
In polymers, intramolecular bonds are due to primary valence bonds (covalent) while the intermolecular
attractions usually are due to secondary bonding forces. The intermolecular forces are opposed by thermal
agitation, which induces vibration, rotation, and translation of a molecular system. Atomic vibrations
exist at all temperature levels. The stability of the molecular system depends on the vibration energy of
the chemical bonds. In polymers, thermal degradation occurs when the energy of vibration exceeds the
primary bonding between atoms, while the transitional phenomena associated with crystalline melting
point, the glass transition temperature, and the polymer deformations are related to rotation and vibration
of molecular chains.
Bearing this in mind, let us consider what happens on a molecular scale when an amorphous polymer
is heated from below its glass transition temperature. At very low temperatures — i.e., in the glassy
state — chain segments are frozen in fixed positions; atoms undergo only low-amplitude vibratory motion
about these positions. As the temperature is increased, the amplitude of these vibrations becomes greater,
thereby reducing the effectiveness of the secondary intermolecular bonding forces. Consequently, the
cooperative nature of the vibrations between neighboring atoms is enhanced. At the glass transition
temperature, chain ends and a substantial number of chain segments have acquired sufficient energy to
overcome intermolecular restraints and undergo rotational and translational motion. Therefore, the glass
transition temperature is referred to as the onset of large-scale cooperative motion of chain segments
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POLYMER SCIENCE AND TECHNOLOGY
(of the order of 20 to 50 consecutive carbon atoms). Rotational and translational modes of motion provide
important mechanisms for energy absorption. This accounts for glassy-to-rubbery transition and the
tough nature of an amorphous polymer above its glass transition temperature.
Below the Tg, or in the glassy state, only atoms or small groups of atoms such as short sections of
the main chain or pendant/side groups move against the local restraints of intermolecular interactions.
This movement may result in other transitions, which are designated α, β, γ, etc., in order of decreasing
temperature. The fully extended chain, which is the conformation of minimum energy, is the preferred
conformation at low temperatures. Therefore, as the molecules straighten out, the free volume, as we
shall see in the next section, decreases. Consequently, flow becomes difficult and the polymer assumes
the characteristic hard and brittle behavior of glasses.
As we said above, the molecular motion of the Tg is restricted only to segmental motion; entire
molecular motion is as yet precluded by chain entanglements. However, above the Tg, or in the rubbery
state, there is a sharp increase in the number of possible conformations. The molecular motion in the
rubbery state requires more free volume, and this rise in the relative free volume leads to the observed
higher volume expansion coefficient above the Tg. As heating is continued into the liquid region,
molecules acquire increased thermal energy, and the amplitudes of associated molecular motions also
increase. Translation, or slip of entire molecules, becomes possible; large changes in conformation occur
and elasticity virtually disappears.
B. THEORIES OF GLASS TRANSITION AND MEASUREMENT
OF THE GLASS TRANSITION TEMPERATURE
The fundamental nature of the glass transition is still unclear. It is a complex process that involves
equilibrium, thermodynamic and kinetic factors. The various theories of the glass transition, however,
have used either the thermodynamic or the kinetic approach. The thermodynamic approach is based on
entropy considerations of the glassy state, while the kinetic theory of the glass transition considers the
relaxation phenomena associated with the glass transition. Each approach gives only a partial explanation
to the observed behavior of polymers. We now briefly discuss these theories along with the free volume
theory.
1. Kinetic Theory
The kinetic concept of glass transition considers the glass transition as a dynamic phenomenon since
the position of the Tg depends on the rate of heating or cooling. It predicts that the value of Tg measured
depends on the time scale of the experiment in relation to that of the molecular motions arising from
the perturbation of the polymer system by temperature changes. A number of models have been proposed
to correlate these molecular motions with changes in macroscopic properties observed in the experiment.
One approach considers the process of vitrification (glassification) as a reaction involving the movement
of chain segments (kinetic units) between energy states. For the movement of a chain segment from one
energy state to another to occur, a critical “hole” or empty space must be available. To create this hole
sufficient energy must be available to overcome both the cohesive forces of the surrounding molecules
and the potential energy barrier associated with the rearrangement. The temperature at which the number
of holes of sufficient size is great enough to permit flow is regarded as the Tg. This theory permits a
description of the approach to thermodynamic equilibrium. When a polymeric material above Tg is
cooled, there is sufficient molecular motion for equilibrium to be achieved. However, the rate of approach
to equilibrium, and hence the Tg, depends on the cooling rate employed in the experiment.
2. Equilibrium Theory
The equilibrium concept treats the ideal glass transition as a true second-order thermodynamic transition,
which has equilibrium properties. The ideal state, of course, cannot be obtained experimentally since
its realization would require an infinite time. According to the theory of Gibbs and DiMarzio,1 the glass
transition process is a consequence of the changes in conformational entropy with changes in temperature.
The reduced level in molecular reorganization observed near the transition temperature is attributed to
the reduction in the number of available conformations as the temperature is lowered. The equilibrium
conformational entropy becomes zero when a thermodynamic second-order transition is reached ultimately. Thereupon, the conformations are essentially “frozen in” since the time required for conformational changes becomes virtually infinite. The glass transition temperature, Tg, therefore approaches the
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THERMAL TRANSITIONS IN POLYMERS
true transition temperature as the time scale of experiment becomes longer. Based on this reasoning and
using a statistical thermodynamics treatment that utilizes a quasi-lattice theory, Gibbs and DiMarzio1
developed quantitative predictions of the second-order phase transition that are in agreement with
experiment.
3. Free Volume Theory
A most useful and popular theory of glass transition is the “free volume” model of Fox and Ferry and,
later, of Williams, Landell, and Ferry.2 This
theory considers the free volume, Vf , of a substance as the
—
difference between its specific volume, V, and the space actually occupied by the molecules, V0, where
V0 is expressed as:
Vo = V ′ + α g T
(4.1)
where V′ = the extrapolated volume of glass at absolute zero
αg = thermal expansion coefficient of the glass
This model further defines the free volume fraction, f, at temperature T as
f = Vf V
(
= fg + α f T − Tg
)
(4.2)
and
α f = α1 − α g =
df
1 dVf
=
dT Vf dT
(4.3)
where fg = free volume fraction at Tg
α1 = thermal expansion coefficient above Tg
αg = thermal expansion coefficient below Tg
For most amorphous polymers, the free volume fraction at the glass transition temperature is found to
be a constant, with a value of 0.025. Amorphous polymers, when cooled, are therefore supposed to
become glassy when the free volume fraction attains this value. Thereupon no significant further change
in the free volume will be observed.
Many important physical properties of polymers (particularly amorphous polymers) change drastically
at the glass transition temperature. The variations of these properties with temperature form a convenient
method for determining Tg. Some of the test methods include the temperature variation of specific volume
(dilatometry) as discussed in Section II, refractive index (refractometry), and specific heat (calorimetry,
DSC or DTA). Others include temperature-induced changes in vibrational energy level (infrared spectroscopy), proton environment (nuclear magnetic resonance or NMR), dipole moment (dielectric constant
and loss), elastic modulus (creep or stress relaxation), and mechanical energy absorption (dynamic
mechanical analysis or DMA). Discussion of details of these test methods is beyond the scope of this
volume.
C. FACTORS AFFECTING GLASS TRANSITION TEMPERATURE
We have seen from the previous discussion that at the glass transition temperature there is a large-scale
cooperative movement of chain segments. It is therefore to be expected that any structural features or
externally imposed conditions that influence chain mobility will also affect the value of Tg. Some of
these structural factors include chain flexibility; stiffness, including steric hindrance, polarity, or interchain attractive forces; geometric factors; copolymerization; molecular weight, branching; cross-linking;
and crystallinity. External variables are plasticization, pressure, and rate of testing.
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POLYMER SCIENCE AND TECHNOLOGY
Table 4.1 Effect of Chain Flexibility on Tg
Polymer
Polyethylene
Repeat Unit
CH2
Th (°C)
CH2
–120
CH3
Polydimethylsiloxane
Si
O
–123
CH3
CH3
Polycarbonate
O
C
O
O
C
150
CH3
CH3
Polysulfone
O
C
CH3
O
O
S
190
O
CH3
Poly(2,6-dimethyl1,4-phenylene oxide)
O
220
CH3
1. Chain Flexibility
Chain flexibility is determined by the ease with which rotation occurs about primary valence bonds.
Polymers with low hindrance to internal rotation have low Tg values. Long-chain aliphatic groups —
ether and ester linkages — enhance chain flexibility, while rigid groups like cyclic structures stiffen the
backbone. These effects are illustrated in Table 4.1. Bulky side groups that are stiff and close to the
backbone cause steric hindrance, decrease chain mobility, and hence raise Tg (Table 4.2).
The influence of the side group in enhancing chain stiffness depends on the flexibility of the group
and not its size. In fact, side groups that are fairly flexible have little effect within each series; instead
polymer chains are forced further apart. This increases the free volume, and consequently Tg drops. This
is illustrated by the polymethacrylate series (Table 4.3).
2. Geometric Factors
Geometric factors, such as the symmetry of the backbone and the presence of double bonds on the main
chain, affect Tg. Polymers that have symmetrical structure have lower Tg than those with asymmetric
structures. This is illustrated by two pairs of polymers: polypropylene vs. polyisobutylene and poly(vinyl
chloride) vs. poly(vinylidene chloride) in Table 4.4. Given our discussion above on chain stiffness, one
would have expected that additional groups near the backbone for the symmetrical polymer would
enhance steric hindrance and consequently raise Tg. This, however, is not the case. This “discrepancy”
is due to conformational requirements. The additional groups can only be accommodated in a conformation with a “loose” structure. The increased free volume results in a lower Tg.
Another geometric factor affecting Tg is cis–trans configuration. Double bonds in the cis form reduce
the energy barrier for rotation of adjacent bonds, “soften” the chain, and hence reduce Tg (Table 4.5).
3. Interchain Attractive Forces
Recall from our earlier discussion that intermolecular bonding in polymers is due to secondary attractive
forces. Consequently, it is to be expected that the presence of strong intermolecular bonds in a polymer
chain, i.e., a high value of cohesive energy density, will significantly increase Tg. The effect of polarity,
for example, can be seen from Table 4.6. The steric effects of the pendant groups in series (CH3, –Cl,
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THERMAL TRANSITIONS IN POLYMERS
Table 4.2 Enhancement of Tg by Steric Hindrance
Polymer
Repeat Unit
Tg(°C)
Polyethylene
CH2
CH2
–120
Polypropylene
CH2
CH
–10
CH3
Polystyrene
CH2
Poly(α-methylstyrene)
Poly(o-methylstyrene)
CH
100
CH3
192
CH2
C
CH2
CH
119
CH3
Poly(m-methylstyrene)
CH2
CH
72
CH3
Poly(α-vinyl naphthalene)
Poly(vinyl carbazole)
CH2
CH2
CH
CH
135
208
N
and –CN) are similar, but the polarity increases. Consequently, Tg is increased in the order shown in the
table. The same effect of increased Tg with increasing CED can be observed when one considers going
from the intermolecular forces in poly(methyl acrylate), an ester, through the strong hydrogen bonds in
poly(acrylic acid) to primary ionic bonds in poly(zinc acrylate) (Table 4.7).
Recall again that secondary bonding forces are effective only over short molecular distances. Therefore, any structural feature that tends to increase the distance between polymer chains decreases the
cohesive energy density and hence reduces Tg. This effect has already been clearly demonstrated in the
polyacrylate series where the increased distance between chains due to the size of the alkyl group, R,
reduced Tg.
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POLYMER SCIENCE AND TECHNOLOGY
Table 4.3 Decrease of Tg with
Increasing Flexibility of Side Chains
for Polymethacrylate Series
Generalized Formula
CH3
CH2
C
C
O
O
R
R
Tg (°C)
methyl
ethyl
n-propyl
n-butyl
n-hexyl
n-octyl
n-dodecyl
105
65
35
21
–5
–20
–65
Table 4.4 Effect of Symmetry of Tg
Polymer
Repeat Unit
Polypropylene
CH2
Tg(°C)
–10
CH
CH3
CH3
Polyisobutylene
CH2
–70
CH
CH3
Poly(vinyl chloride)
CH2
87
CH
Cl
Cl
Poly(vinylidene chloride)
CH
–17
C
Cl
Table 4.5 Relative Effects of cis–trans Configuration on Tg
Polymer
Poly(1,4-cis-butadiene)
Repeat Unit
CH2
CH2
CH
Poly(1,4-trans-butadiene)
Tg(°C)
CH
–83
CH2
CH
CH
CH2
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THERMAL TRANSITIONS IN POLYMERS
Table 4.6 Effect of Polarity on Tg
Polymer
Repeat Unit
Polypropylene
CH2
Dielectric
Constant at 1kHz
Tg(°C)
2.2–2.3
–10
CH
CH3
Poly(vinyl chloride)
CH2
CH
3.39
87
5.5
103
Cl
Polyacrylonitrile
CH2
CH
CN
Table 4.7 Effect of Polarity on the Tg
of Some Acrylic Polymers
Polymer
Repeat Value
Polymethylacrylate
CH2
3
CH
C
Tg(°C)
O
O
CH3
Poly(acrylic acid)
CH2
106
CH
C
O
O
H
Poly(zinc acrylate)
CH2
>400
CH
C
O
O
Zn++
O
O
C
CH2
CH
4. Copolymerization
The transition temperatures Tg and Tm are important technological characteristics of polymers. It is
desirable — in fact, valuable — to be able to control either Tg or Tm independent of each other. This,
however, is often impossible. Polymer chemists have circumvented this problem to some extent by
polymer modification via copolymerization and polyblending. These procedures have become powerful
tools for tailoring polymer systems for specific end uses.
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POLYMER SCIENCE AND TECHNOLOGY
Figure 4.4 Er vs. fraction ratios of butadiene–styrene copolymers. (From Tobolsky, A.V., Properties and Structure
of Polymers, John Wiley & Sons, New York, 1960. With permission of Dorothy Tobolsky.)
A copolymer system may be characterized either by the geometry of the resulting polymer — that
is, the arrangement of the different monomers (random, alternating, graft, or block) — or by the
compatibility (miscibility) of the two monomers.
a. Isomorphous Systems (Homogeneous Copolymers
or Compatible Polyblends)
In isomorphous systems, the component monomers occupy similar volumes and are capable of replacing
each other in the crystal system. The resulting copolymer, irrespective of its geometry, is necessarily
homogeneous, and polyblends of the individual homopolymers or copolymers have similar transition properties. Copolymerization merely shifts the Tg to the position intermediate between those of the two homopolymers; it does not alter the temperature range or the modulus within the transition region (Figure 4.4). This
shift is illustrated in Figure 4.4, which shows the modulus temperature curves for polybutadiene (100/0) and
polystyrene (0/100) and for various compositions of butadiene–styrene copolymer.
For this system, if the glass transitions (Tg1 and Tg2) of the individual homopolymers (1 and 2) are
known, it is possible to estimate the Tg of the copolymer (or polyblend) using the relation
Tg = V1 Tg 2 + V2 Tg 2
(4.4)
where V1 and V2 are the volume fractions of components 1 and 2, respectively. This is shown schematically in Figure 4.5 (line 1).
b. Nonisomorphous Systems
In nonisomorphous systems, the specific volumes of the monomers are different. In this case, the
geometry of the resulting polymer becomes important.
Random or alternating — For these copolymers, the composition is necessarily homogeneous (no
phase separation) and, as discussed above, the glass transitions are intermediate between those of the
two homopolymers. The increased disorder resulting from the random or alternating distribution of
monomers enhances the free volume and consequently reduces Tg below that predicted by Equation 4.4
(line 2, Figure 4.5). The Tg of the copolymer whose components have weight fractions W1 and W2 and
glass transitions Tg1 and Tg2, respectively, can be calculated from the relation
1 W1 W2
=
+
Tg Tg1 Tg2
(4.5)
Examples of this type are methyl methacrylate–acrylonitrile, styrene–methyl methacrylate, and acrylonitrile–acrylamide copolymers. It is also possible that monomers involved in the copolymerization process
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THERMAL TRANSITIONS IN POLYMERS
Figure 4.5 Variation in glass transition temperature with copolymer composition (schematic).
(as in the copolymers methylacylate–methylmethacrylate and vinylidene chloride–methylacrylate) introduce significant interaction between chains. In this case the Tg will be enhanced relative to the predicted
value (Figure 4.5, line 3).
Block and graft copolymers (incompatible copolymers) — For block or graft copolymers in which
the component monomers are incompatible, phase separation will occur. Depending on a number of
factors — for example, the method of preparation — one phase will be dispersed in a continuous matrix
of the other. In this case, two separate glass transition values will be observed, each corresponding to
the Tg of the homopolymer. Figure 4.6 shows this behavior for polyblends of polystyrene (100) and
30/70 butadiene–styrene copolymer (0).
Example 4.1: What is the Tg of butadiene–styrene copolymer containing 10 vol% styrene?
Solution: Butadiene and styrene form a completely compatible random copolymer. Therefore the following relation is applicable: Tg = V1Tg1 + V2Tg2.
Assume 1 = polybutadiene
2 = polystyrene
Tg1 = −80°C, Tg 2 = 100°C
Tg = 0.90 (−80) + 0.10 (100)
= −62°C
5. Molecular Weight
Since chain end segments are restricted only at one end, they have relatively higher mobility than the
internal segments, which are constrained at both ends. At a given temperature, therefore, chain ends
provide a higher free volume for molecular motion. As the number of chain ends increases (which means
a decrease in Mn), the available free volume increases, and consequently there is a depression of Tg. The
effect is more pronounced at low molecular weight, but as Mn increases, Tg approaches an asymptotic
value. An empirical expression relating the inverse relations between Tg and Mn is given by Equation 4.6.
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POLYMER SCIENCE AND TECHNOLOGY
Figure 4.6 Er(10) vs. temperature for polyblends of polystyrene and a 30/70 butadiene–styrene copolymer.
Numbers on the curves are the weight percent of polystyrene in the blend. (From Tobolsky, A.V., Properties and
Structure of Polymers, John Wiley & Sons, New York, 1960. With permission of Dorothy Tobolsky.)
Tg = Tg∞ = K M n
(4.6)
where T∞g = Tg of an infinite molecular weight
K = a constant
For polystyrene it has been found that T∞g = 100°C while K is about 2 × 105.
Example 4.2: What is the Tg of polystyrene of Mn = 3000?
Solution: From above, T∞g = 100, K = 2 × 105.
Tg = 100 −
2 × 10 5
3000
= 33°C
6. Cross-Linking and Branching
By definition, cross-linking involves the formation intermolecular connections through chemical bonds.
This process necessarily results in reduction in chain mobility. Consequently, Tg increases. For lightly
cross-linked systems like vulcanized rubber, Tg shows a moderate increase over the uncross-linked
polymer. In this case, Tg and the degree of cross-linking have a linear dependence, as shown by the
following approximate empirical equation.
Tg − T 0 =
3.9 × 10 4
Mc
(4.7)
where Tg = the glass transition temperature of the uncross-linked polymer having the same chemical
composition as the cross-linked polymer
Mc = the number-average molecular weight between cross-linked points
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THERMAL TRANSITIONS IN POLYMERS
107
For highly cross-linked systems like phenolics and epoxy resins, the glass transition is virtually infinite.
This is because the molecular chain length between cross-links becomes smaller than that required for
cooperative segmental motion.
Like long and flexible side chains, branching increases the separation between chains, enhances the
free volume, and therefore decreases Tg.
7. Crystallinity
In semicrystalline polymers, the crystallites may be regarded as physical cross-links that tend to reinforce
or stiffen the structure. Viewed this way, it is easy to visualize that Tg will increase with increasing
degree of crystallinity. This is certainly not surprising since the cohesive energy factors operative in the
amorphous and crystalline regions are the same and exercise similar influence on transitions. It has been
found that the following empirical relationship exists between Tg and Tm.
1 2 for symmetrical polymers
=
Tm 2 3 for unsymmetrical polymers
Tg
(4.8)
where Tg and Tm are in degrees Kelvin.
8. Plasticization
Plasticity is the ability of a material to undergo plastic or permanent deformation. Consequently,
plasticization is the process of inducing plastic flow in a material. In polymers, this can be achieved in
part by the addition of low-molecular-weight organic compounds referred to as plasticizers (see
Chapter 9). Plasticizers are usually nonpolymeric, organic liquids of high boiling points. Plasticizers are
miscible with polymers and, in principle, should remain within the polymer. Addition of plasticizers to
a polymer, even in very small quantities, drastically reduces the Tg of the polymer. This is exemplified
by the versatility of poly(vinyl chloride) which, if unmodified, is rigid, but can be altered into a flexible
material by the addition of plasticizers such as dioctylphthalate (DOP).
The effect of plasticizer in reducing Tg can be interpreted in several ways. Plasticizers function through
a solvating action by increasing intermolecular distance, thereby decreasing intermolecular bonding
forces. Alternatively, the addition of plasticizers results in a rapid increase in chain ends and hence an
increase in free volume. A plasticized system may also be considered as a polyblend, with the plasticizer
acting as the second component. In this case, our earlier relations for polyblends would apply
(Equations 4.1 and 4.2). Since plasticizers generally have very low Tg, between –50°C and –160°C,
addition of small amounts of the plasticizer would be expected to result in a substantial decrease in the
Tg of a polymer. This is illustrated in Figure 4.7 for a poly(vinyl chloride)–diethylhexyl succinate system.
Figure 4.7 Shear modules, G vs. temperature, measured for a time scale of approximately 1 s, poly(vinyl
chloride) plasticized with diethylhexyl succinate. I, 100% monomer; II, 91%; III, 79%; IV, 70.5%; V, 60.7%; VI,
51.8%; VII, 40.8%. (From Schneider, K. and Wolf, K., Kolloid Z., 127, 65, 1952.)
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POLYMER SCIENCE AND TECHNOLOGY
Observe that in addition to the reduction in Tg, there is a considerable broadening of the transition region
in plasticized PVC.
Example 4.3: Toluene behaves as a plasticizer for polystyrene. Estimate Tg of a polystyrene sample
containing 20 vol% toluene.
Solution: Since toluene is completely compatible with polystyrene, we may use the relation
Tg = VA TgA + VBTgB
where TgA, TgB = glass transition temperature of polystyrene and toluene, respectively
VA, VB = volume fraction of polystyrene and toluene, respectively
Assuming Tga = melting point of toluene = –50°C.
( )
Tg = (1 − VB ) TgA + VB TgB
= 0.8 × 100 × 0.2 (−50)
= 70°C
III.
THE CRYSTALLINE MELTING POINT
Melting involves a change from the crystalline solid state into the liquid form. For low-molecular-weight
(simple) materials, melting represents a true first-order thermodynamic transition characterized by
discontinuities in the primary thermodynamic variables of the system such as heat capacity, specific
volume (density), refractive index, and transparency. Melting occurs when the change in free energy of
the process is zero; that is,
∆G m = ∆H m − Tm ∆g m = 0
(4.9)
or
Tm =
∆H m
∆Sm
(4.10)
where ∆Hm = enthalpy change during melting and represents the difference between cohesive energies
of molecules in the crystalline and liquid states
∆Sm = entropy change during melting representing the change in order between the two states
This concept has been extended to melting in crystalline polymeric systems. We must remember, however,
that in the case of crystalline polymers:
• The macromolecular nature of polymers and the existence of molecular weight distribution (polydispersity) lead to a broadening of Tm.
• The process of crystallization in polymers involves chain folding. This creates inherent defects in the
resulting crystal. Consequently, the actual melting point is lower than the ideal thermodynamic melting
point.
• Because of the macromolecular nature of polymers and the conformational changes associated with
melting, the process of melting in polymer is more rate sensitive than that in simple molecules.
• No polymer is 100% crystalline.
The factors that determine crystallization tendency have been dealt with earlier (Chapter 3). We simply
recap them here.
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THERMAL TRANSITIONS IN POLYMERS
• Structural regularity — For the effective utilization of the secondary intermolecular bonding forces
during the formation of a crystalline polymer, close alignment of polymer molecules is a prerequisite.
Any structural feature of a polymer chain that impedes this condition must necessarily detract from
crystallinity.
• Chain flexibility — In the process of aggregation to form a crystalline solid, polymer molecules are
opposed by thermal agitation, which induces segmental rotational and vibrational motion. Polymers
with flexible chains are more susceptible to this agitation than those with stiff backbones. Consequently,
chain flexibility reduces the tendency for crystallization.
• Intermolecular bonding — Since secondary bonding forces are responsible for intermolecular bonding,
polymer molecules with specific groups that promote enhanced intermolecular interaction and whose
structural features lead to identity periods are more crystallizable.
As we said earlier, melting is a true first-order thermodynamic transition involving a phase change
and is associated with discontinuities in primary extensive thermodynamic properties. In principle,
therefore, any property whose values are different for the crystalline and amorphous states provides a
convenient method for measuring the crystalline melting point. Methods for measuring the crystalline
melting point include dilatometry, calorimetry, and thermal analysis; dynamic techniques (mechanical
dielectric, nuclear magnetic resonance); stress relaxation; and creep.
A. FACTORS AFFECTING THE CRYSTALLINE MELTING POINT, TM
Bearing in mind the peculiar nature of polymers, melting in crystalline polymers can be considered a
pseudoequilibrium process that may be described by the free energy equation:
Tm =
∆H m
∆Sm
(4.10)
In this case, ∆Hm represents the difference in cohesive energies between chains in the crystalline and
liquid states, while ∆Sm represents the difference in the degree of order between polymer molecules in
the two states. ∆Hm is generally independent of the molecular weight. But, as would be expected, polar
groups on the chain — particularly if disposed regularly on the chain so as to encourage regions of
extensive cooperative bonding — would enhance the magnitude of ∆Hm. ∆Sm depends not only on
molecular weight, but also on structural factors like chain stiffness. Chains that are flexible in the molten
state would be capable of assuming a relatively larger number of conformations than stiff chains and
hence result in a large ∆Sm. We now discuss these factors that affect Tm in greater detail.
1. Intermolecular Bonding
The cohesive forces in polymers involve the secondary bonding forces ranging from the weak van der
Waals forces through the much stronger hydrogen bonds. In some cases, these forces even include
primary ionic bonds. Figure 4.8 shows the variation of Tm for a homologous series of various types of
polymers. With polyethylene as a reference and neglecting for the moment possible fine details in trends,
we observe that:
• The melting points approach that of polyethylene as the spacing between polar groups increases.
• For the same number of chain atoms in the repeat unit, polyureas, polyamides, and polyurethanes have
higher melting points than polyethylene, while polyesters have lower.
As would be expected, the decrease in the cohesive energy density associated with the decrease in
the density of sites for intermolecular bonding (increased space between polar groups) leads to a reduction
in the melting points.
Van Krevelen and Hoftyzer6 have calculated the contributions of the characteristic groups in various
polymers to Ym, a quantity they termed molar melt transition function (identified with ∆Hm in
Equation 4.7). These are shown in Table 4.8. For the same number of chain backbone atoms, chain
flexibility (∆Sm) will not be significantly different for the various polymers. From Table 4.8, while the
absolute values calculated for the characteristic interunit groups may not be significant, the trend in their
magnitudes definitely corresponds to that of the melting points of the various types of polymers. In
Figure 4.8, we note specifically that the melting points for polyesters are lower than the Tm of polyethylene.
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POLYMER SCIENCE AND TECHNOLOGY
Figure 4.8 Trend of crystalline melting points in homologous series of aliphatic polymers. (From Hill, R.H. and
Walker, E.E., J. Polym. Sci., 3, 609, 1948. With permission.)
Table 4.8 Group Contributions to the Melting
Point
Polymer
Characteristic
Group
Contribution to Ym
O
Polyester
Polyamide
Polyurethane
Polyurea
C
O
O
H
C
N
1160
2560
O
H
O
C
N
H
O
H
N
C
N
2430
3250
From Van Krevelen, D.W. and Hoftyzer, P.J., Properties of
Polymers: Correlations with Chemical Structure, Elsevier,
Amsterdam, 1976. With permission.
This is because the enhanced flexibility resulting from the presence of oxygen atoms in the polyester
chains considerably offsets the weak polar bonding from ester linkages. This is demonstrated in Table 4.9.
The melting points of the nylons reflect the density of the hydrogen-bond-forming amide linkages. The
densities of the interunit linkages in polycaprolactone (ester units) and polycaprolactam (amide units)
are the same. However, the amide units are more polar than the ester units. Consequently, polycaprolactam
has a much higher Tm than polycaprolactone.
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THERMAL TRANSITIONS IN POLYMERS
Table 4.9 Effect of Intermolecular Bonding on Tm
Polymer
Characteristic Group
Melting Temperature (Tm)(°C)
O
Polycaprolactone
O
(CH2)5
O
Polycaprolactam (nylon 6)
C
C
(CH2)5
(CH2)4
O
Nylon 12
C
n
H
O
Poly(hexamethylene adipamide)
(nylon 6,6)
61
C
226
N
n
O
H
C
N
H
(CH2)6
N
n
265
H
(CH2)11
179
N
n
Figure 4.9 Relation between Tm and Tg for various polymers. (From Bayer, R.F., J. Appl. Phys., 25, 585, 1954.
With permission.)
2. Effect of Structure
The structural dependence of the crystalline melting temperature is essentially the same as that for the
glass transition temperature. The only difference is the effect of structural regularity, which has a profound
influence on crystallizability of a polymer. Tg is virtually unaffected by structural regularity. From a
close examination of data for semicrystalline polymers it has been established that the ratio Tg /Tm (K)
ranged from 0.5 to 0.75. The ratio is found to be closer to 0.5 in symmetrical polymers (e.g., polyethylene
and polybutadiene) and closer to 0.75 in unsymmetrical polymers (e.g., polystyrene and polychloroprene). This behavior is shown in Figure 4.9.
3. Chain Flexibility
Polymers with rigid chains would be expected to have higher melting points than those with flexible
molecules. This is because, on melting, polymers with stiff backbones have lower conformational entropy
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112
POLYMER SCIENCE AND TECHNOLOGY
changes than those with flexible backbones. As we saw earlier, chain flexibility is enhanced by the
presence of such groups as –O– and –(CO·O)– and by increasing the length of (–CH2–) units in the
main chain. Insertion of polar groups and rings restricts the rotation of the backbone and consequently
reduces conformational changes of the backbone, as illustrated by the following polymers (Table 4.10).
Table 4.10 Effect of Chain Flexibility to Tm
Polymer
Repeat Unit
Tm(°C)
Polyethylene
CH2
CH2
135
Polypropylene
CH2
CH
165
CH3
Polyethylene oxide
CH2
CH2
O
66
Poly(propylene oxide)
CH2
CH
O
75
CH3
O
Poly(ethylene adipate)
Poly(ethylene terephthalate)
Poly (diphenyl-4,-4 diethylene
carboxylate)
O
O
O
CH2
CH2
CH2CH2
CH2CH2
O
O
O
C
O
(CH2)4
O
O
C
C
O
265
O
O
C
C
CH3
Polycarbonate
50
C
C
355
O
O
C
270
CH3
Poly(p-xylene)
CH2
Polystyrene (isotactic)
CH2
CH
Poly(o-methylstyrene)
CH2
CH
CH2
CH
Poly(m-methylstyrene)
CH2
240
>360
CH3
215
CH3
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380
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THERMAL TRANSITIONS IN POLYMERS
Figure 4.10 Melting points of copolymers of hexamethylene adipamide and terephthalamide, and of hexamethylene sebacamide and terephthalamide. (From Edgar, D.B. and Hill, R.J., J. Polym. Sci., 8(1), 1, 1952. With
permission.)
4. Copolymerization
The effect of copolymerization on Tm depends on the degree of compatibility of the comonomers. If the
comonomers have similar specific volumes, they can replace each other in the crystal lattice (i.e.,
isomorphous systems), and the melting point will vary smoothly over the entire composition range. On
the other hand, if the copolymer is made from monomers each of which forms a crystalline homopolymer,
the degree of crystallinity and the crystalline melting point decrease as the second constituent is added
to either of the homopolymers. In this case, the Tm of the copolymer (i.e., the reduction in the melting
point, T°m of the homopolymer due to the addition of the second constituent) is given by Equation 4.11.8
1
1
R
=
−
ln x
Tm
Tm° ∆H m
(4.11)
where ∆Hm and X are, respectively, the heat of fusion and mole fraction of the homopolymer or
crystallizing (i.e., major) component (Figure 4.10). It is obvious from the foregoing that it is impossible
to attempt to raise the crystalline melting point of a polymer by copolymerizing with small amounts of
a monomer with a high melting point except for isomorphous systems, which are rare in vinyl polymers.
Block and graft copolymers with sufficiently long homopolymer chain sequences crystallize and
exhibit properties of both homopolymers and consequently have two melting points, one for each type
of chain segment.
Example 4.4: What is the melting point of a copolymer of ethylene and propylene with 90 mol%
ethylene?
Solution: From the foregoing discussion:
1
1
R
=
−
ln x
Tm Tm° ∆H m
Tm° = melting point of PE
= 135°C = 408 K
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POLYMER SCIENCE AND TECHNOLOGY
∆H m for PE = 66.5 cal g = 7.80 × 103 J mol
(Note molecular weight of PE [repeat unit] = 28)
1
1
8.314
=
−
ln 0.9
Tm 408 7.80 × 103
= 25.63 × 10 −4
Tm = 390 K = 117°C
IV.
PROBLEMS
4.1. Arrange the following materials in the probable order of their increasing crystalline melting points and
justify your answer. Assume the degree of polymerization, n, for the polymers is the same.
H
a.
N
(CH2)6
H
b.
N
(CH2)6
H
c.
N
(CH2)6
H
N
d.
(CH2)6
H
O
N
C
H
O
N
C
O
(CH2)8
O
(CH2)6
(CH2)4
H
O
H
N
C
N
H
O
H
N
C
N
O
C
n
O
C
n
CH2
CH2
(CH2)4
O
e.
C
H
O
N
C
H
O
N
C
n
n
O
(CH2)8
C
n
4.2. The solubility parameters of poly(vinyl chloride) (PVC) and dibutyl sebacate are 9.7 and 9.2, respectively.
What amount (in volume percent) of dibutyl sebacate will be required to make PVC a flexible polymer
at room temperature? Assume that the Tg of dibutyl sebacate is –100°C and that room temperature is 25°C.
4.3. Arrange the following linear polymers in orders of decreasing crystalline melting points. Explain the
basis of your decision.
a.
b.
c.
d.
Poly(ethylene adipate)
Poly(ethylene terephthalate)
Poly(hexamethylene adipate)
Poly(ethylene adipamide)
4.4. Cross-linking polystyrene with divinyl benzene increased its Tg by 7.5°C. What is the number of styrene
residues between cross-links?
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THERMAL TRANSITIONS IN POLYMERS
4.5. The Tg of polystyrene is 100°C. What is its melting temperature, Tm?
4.6. The heat of fusion of the repeating unit for a homopolymer that melts at 250°C is 2500 cal/mol. Predict
the melting point of a random copolymer of this polymer with 25 mol% of a comonomer.
4.7. Which of the following pairs of polymers will have a higher glass transition temperature, Tg? Explain
your choice.
a. Poly(2-chloroethyl methacrylate) or poly(n-propyl methacrylate)
b. Poly(n-butyl methacrylate) or poly(2-methoxyethyl methacrylate)
4.8. What is the free volume fraction of polystyrene at 150°C if its volume coefficient of expansion is 60.0 ×
10–6 cm/cm3 °C?
4.9. Which of each of the following pairs has a higher Tµ? Why?
a. Polyethylene or a random copolymer of polyethylene and polypropylene
b. Poly(vinyl chloride) or polytetrafluoroethylene
c. Nylon 6 or nylon 11
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
Gibbs, J.H. and DiMarzio, E.A., J. Chem. Phys. 28, 373, 1955; 28, 807, 1958.
Williams, M.L., Landell, R.F., and Ferry, J.A., J. Am Chem. Soc., 77, 3701, 1955.
Tobolsky, A.V., Properties and Structure of Polymers, John Wiley & Sons, New York, 1960.
Schneider, K. and Wolf, K., Kolloid Z., 127, 65, 1952.
Hill, R.H. and Walker, E.E., J. Polym. Sci., 3, 609, 1948.
Van Krevelen, D.W. and Hoftyzer, P.J., Properties of Polymers: Correlations with Chemical Structure, Elsevier,
Amsterdam, 1976.
Bayer, R.F., J. Appl. Phys., 25, 585, 1954.
Flory, P.J., Principles of a Polymer Chemistry, Cornell University Press, Ithaca, New York, 1953, chap. 13.
Edgar, O.B. and Hill, R.J., J. Polym. Sci., 8(1), 1, 1952.
Fried, J.R., Plast. Eng., 38(7), 27, 1982.
Chruma, J.L. and Chapman, R.D., Chem. Eng. Prog., 8, 49, 1985.
Billmeyer, F.W., Jr., Textbook of Polymer Science, 3rd ed., John Wiley & Sons, New York, 1984.
Williams, D.J., Polymer Science and Engineering, Prentice-Hall, Englewood Cliffs, NJ, 1971.
Kaufman, H.S. and Falcetta, J.J., eds., Introduction to Polymer Science and Technology, John Wiley & Sons, New
York, 1977.
Copyright 2000 by CRC Press LLC
Chapter 5
Polymer Modification
I.
INTRODUCTION
In Chapter 2, we discussed the various polymerization mechanisms. In principle, with the large number
of monomers available, a virtually endless number of polymers can be obtained by various polymerization
reactions. Quite frequently, a given monomer is converted into the corresponding homopolymer. However, in order to meet increasingly stringent and specific end-use requirements, novel polymeric materials
have also been developed through modification of existing polymers. Development of tailor-made
materials by polymer modification is usually less costly and can be achieved more readily than through
synthesis of new polymers.
For a polymer to be useful, it must be able to function properly in a given application. The performance
of a polymer is determined primarily by the composition and structure of the polymer molecule. These
control the physical, chemical, and other characteristics of the polymer material. Therefore modification
of the composition of the structural units represents one of the main approaches to the modification of
polymer behavior. In addition to the chemical nature and composition of the structural units that constitute
the polymer backbone, molecular architecture also contributes to the ultimate properties of polymeric
products. Thus polymer modification can be accomplished by employing one or more of the following
techniques:
• Copolymerization of more than one monomer
• Control of molecular architecture
• Postpolymerization polymer reactions involving functional/reactive groups introduced deliberately into
the polymer main chain or side groups
The above property modification techniques are associated with the control of the chemical, composition, and structural nature of the polymer, which is effected largely during the polymerization process.
However, few polymers are used technologically in their chemically pure form. Virtually all commercially
available polymeric materials are a combination of one or more polymeric systems with various additives
designed, with due consideration to cost factors, to produce an optimum property and/or process profile
for specific applications. Modification of polymers through the use of chemical additives and reinforcements through alloying and blending procedures and by composite formation is discussed in Chapter 9.
In this chapter, we restrict our discussion to polymer modification techniques associated with chemical
phenomena brought about either during or after polymerization.
II.
COPOLYMERIZATION
Macromolecular design and architecture through copolymerization of monomers has led to a number of
commercially important polymers. Copolymer composition can be varied over wide limits, resulting in
a wide range of property/process performance. A copolymer may be composed of comparable amounts
of the constituent monomers. The properties of the resulting copolymer will be substantially different
from those of the parent homopolymers. On the other hand, the copolymer may contain only a very
small amount of one of the monomers. In this case, the gross physical characteristics of the copolymer
probably approximate those of the homopolymer of the major constituent, while the minor constituent
confers specific chemical properties on the copolymer. We now illustrate these principles by discussing
some examples.
A. STYRENE–BUTADIENE COPOLYMERS
Polybutadiene is an elastomeric material with good elastic properties and outstanding toughness and
resilience. However, it has relatively poor resistance to oils, solvents, oxidation, and abrasion. Polystyrene, on the other hand, is relatively chemically inert and is quite resistant to alkalis, halide acids, and
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POLYMER SCIENCE AND TECHNOLOGY
oxidizing and reducing agents. It is also very easy to process. But then, polystyrene is quite brittle with
a low heat-deflection temperature (82 to 88°C). Styrene and butadiene copolymers provide a good
illustration of the considerable latitude in the variation of polymer properties that can be achieved by a
careful manipulation of the composition of the copolymer and the distribution of these components.
Styrene and butadiene can be copolymerized to produce either random, graft, or block copolymers.
Styrene–butadiene random copolymer exhibits a homogeneous single phase and has properties intermediate to those of the parent homopolymer. On the other hand, block or graft copolymers of styrene and
butadiene form heterogeneous multiphase systems whose properties are not simply characteristic of each
homopolymer, but are dictated by the multiphase character of the copolymer.
Most of the defects of polybutadiene homopolymer can be overcome by the incorporation of 28%
styrene into the copolymer. This is due essentially to the great rigidity and other beneficial properties
of the styrene molecule. The enhanced properties of SBR and its ease of processability makes SBR the
preferred material over natural rubber in such applications as belting, hose, and molded goods and in
vulcanized sheet and flooring. Rubber shoe soles are made almost exclusively from SBR. Copolymers
containing about 25% styrene are also useful adhesives, especially in the form of aqueous dispersion or
in solution. If the ratio of styrene to butadiene is in the range 60:40 and higher, the copolymer is nontacky
and is used in hot-melt adhesives and latex paints. For example, emulsion copolymers composed of 74%
styrene and 25% butadiene (by weight) find wide applications in paints. In those applications, the
hardness of polystyrene is partially retained, but its brittleness is modified by the presence of butadiene.
1. Styrene–Butadiene Rubber (SBR) (Random Copolymer)
SBR is produced by the free-radical polymerization of styrene and butadiene, and, as such, the resulting
copolymer is necessarily random. Also, the structure is irregular. Consequently, SBR is not crystallizable.
Butadiene can undergo either 1,2 or 1,4 polymerization. The structure of the resulting SBR copolymer
is shown in Figure 5.1.
Commercial SBR is produced by either emulsion or solution copolymerization of butadiene and
styrene. Emulsion copolymerization is either a cold (41°F) or hot (122°F) process. The copolymers from
the hot and cold processes have principal differences in molecular weight, molecular weight distribution,
and microstructure, as shown in Table 5.1. The solution copolymerization process for the production of
CH2
CH
CH2
CH
CH2
CH2
CH
CH
CH2
y
x
z
Figure 5.1 Structure of styrene–butadiene rubbers.
Table 5.1 Property Differences between Hot
and Cold SBR
Property
Molecular weight
—
Viscosity average, M v
—
Weight average, M w
—
Number average, M n
Microstructure (%)
1,4 (cis)
1,4 (trans)
1,2 (vinyl)
Hot
Cold
150–400,000
250–450,000
30–100,000
280,000
500,000
110–260,000
15
58
27
18
69
23
From Stricharczuk, P.T. and Wright, D.E., Handbook of
Adhesives, 2nd ed., Skeist, I., Ed., Van Nostrand Reinhold,
New York, 1977. With permission.
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POLYMER MODIFICATION
119
SBR involves the use of alkyllithium catalysts. Solution SBR generally has a higher molecular weight,
narrower molecular weight distribution, and higher cis-diene content than emulsion SBR.
Example 5.1: Generally, hot SBR is better suited to adhesive formulation than cold SBR. Explain.
Solution: From Table 5.1, hot SBR has a lower molecular weight and a broader molecular weight
distribution than cold SBR. The lower molecular weight fraction provides “quick stick,” while the higher
molecular weight fraction provides shear strength. On the other hand, narrow molecular weight distribution and higher trans-structure of cold SBR make it less adaptable to adhesives.
Example 5.2: Explain the following observations:
a. Tire treads made from solution SBR are superior to those from emulsion SBR.
b. The ozone resistance of SBR is superior to that of natural rubber, but when cracks start in SBR
they grow much more rapidly.
Solution:
a. Solution SBR has a higher molecular weight, narrower molecular weight distribution, and higher
cis-1,4-polybutadiene content than emulsion SBR. These qualities reduce tread wear and enhance
crack resistance of the tire.
b. The presence of styrene in SBR reduces the amount of unsaturation relative to natural rubber.
Consequently, the ozone resistance of SBR is higher than that of natural rubber. Natural rubber
has a cis-1,4 configuration, which makes it a tough material resistant to crack growth. SBR is
mainly trans-1,4, which is less impact resistant.
2. Styrene–Butadiene Block Copolymers
Styrene–butadiene block copolymers belong to a new class of polymers called thermoplastic elastomers
(TPE). Products made from these polymers have properties similar to those of vulcanized rubbers, but
they are made from equipment used for fabricating thermoplastic polymers. Vulcanization is a slow and
energy-intensive thermosetting process. In contrast, the processing of thermoplastic elastomers is rapid
and involves cooling the melt into a rubberlike solid. In addition, like true thermoplastics, scrap from
TPE can be recycled.
Styrene–butadiene block copolymer belongs to the A-B-A type thermoplastic elastomer. The principal
structure of this type of polymer involves the thermoplastic rubber molecules terminated by the hard,
glassy end blocks. The A and B copolymer block segments are incompatible and, consequently, separate
spontaneously into two phases. Thus in the solid state, the styrene–butadiene (S-B-S) thermoplastic
elastomer has two phases: a continuous polybutadiene rubber phase and the dispersed glassy domains
of polystyrene. The styrene plastic end blocks, called domains, act as cross-links locking the rubber
phase in place.
In commercial thermoplastic S-B-S rubber, the end-block phase is present in a smaller proportion
with a styrene-to-butadiene (end-block-to-midblock) ratio in the range 15:85 to 40:60 on weight basis.
The useful temperature range of S-B-S copolymer lies between the Tg of polybutadiene and polystyrene.
Below the Tg of polybutadiene, the elastomeric midblocks become hard and brittle. Above the Tg of
polystyrene, the domains soften and cease to act as cross-links for the soft midblocks. Between the Tg
of both homopolymers, however, the hard styrene domains prevent the flow of the soft elastomeric
butadiene midsegments through a network similar to vulcanized rubber. Therefore, within normal use
temperature, S-B-S block copolymer retains the thermoplasticity of styrene blocks and the toughness
and resilience of the elastomer units.
B. ETHYLENE COPOLYMERS
Low-density polyethylene (LDPE) is produced under high pressures and temperatures. It finds applications in film and sheeting uses and in injection-molded products such as insulated wires and cables. Its
physical properties are dictated by three structural variables: density, molecular weight, and molecular
weight distribution. As density increases, barrier properties, hardness, abrasion, heat, and chemical
resistance, strength, and surface gloss increase. On the other hand, decreasing density results in enhanced
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POLYMER SCIENCE AND TECHNOLOGY
Table 5.2 Some Ethylene Copolymers
R
Copolymer
O
C
Ethylene–methyl acrylate (EMA)
O
Me
O
Et
Ethylene–ethyl acrylate (EEA)
Me
Ethylene–vinyl acetate (EVAc)
O
C
O
O
C
toughness, stress-crack resistance, clarity, flexibility, and elongation and in reduced creep and mold
shrinkage. Melt index (MI) is a measure of molecular weight; it decreases with increasing molecular
weight. Increasing MI improves clarity, surface gloss, and mold shrinkage. Decreasing MI leads to
improved creep and heat resistance, toughness, melt strength, and stress-cracking. Narrower molecular
weight distribution gives better impact strength, but reduced resin processability. A broader molecular
weight distribution is more shear sensitive and, consequently, leads to shear viscosity at high shear rates
and is thus easier to process.
By copolymerizing ethylene with polar α-olefins, it is possible to produce a variety of materials
ranging from rubbery to low melting products — suitable for hot-melt adhesives — to those that
demonstrate unusual toughness and flexibility. This class of copolymers can be represented by the general
formula
CH2
CH2
x
CH2
CH
y
R
where R is a polar group shown in Table 5.2. The introduction of comonomers with a polar pendant
group, R, produces a highly branched random copolymer but with increased interchain interaction. Thus,
relative to the homopolymer, these copolymers have enhanced flexibility, toughness, stress-cracking
resistance, oil and grease resistance, clarity, and weatherability. Some of these are illustrated for EVAc
in Table 5.3. EEA, for example, also has the flexibility of plasticized vinyl without the thermal instability
and plasticizer migration problems associated with PVC. In general, the range of properties of the
copolymer can be varied by varying the proportion and molecular weight of the comonomer.
C. ACRYLONITRILE-BUTADIENE-STYRENE (ABS)
ABS is the generic name of a family of engineering thermoplastics produced by a combination of three
monomers: acrylonitrile, butadiene, and styrene. The overall property balance of the terpolymer is a
Table 5.3 Properties of Rotational-Molding Polyethylene Resins
Property
Melt index, g/10 min
Density, g/cm3
Tensile strength, psi
Elongation, %
Flexural modulus, psi
Environmental stress-crack resistance, h
Tensile impact strength, ft-lb/in.2
Dart impact strength at –20°F, ft-lb
EVAc Copolymer
LDPE
1.5
0.933
1600
500
50,000
50
32
32
1.5
0.925
1450
450
35,000
20
25
20
From Wake, W.C., Adhesion and the Formulation of Adhesives, Applied Science
Publishers, London, 1976.
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POLYMER MODIFICATION
121
result of the contribution of the unique characteristics of each monomer. Polymer chemical resistance
and heat and aging stability depend on acrylonitrile, while its toughness, impact resistance, and property
retention at low temperature are developed through butadiene. Copolymer rigidity, glossy surface appearance, and ease of processability are contributions from styrene. The terpolymer properties are controlled
by manipulation of the ratio and distribution of the three components.
ABS resins consist essentially of two phases: a rubbery phase dispersed in a continuous glassy matrix
of styrene–acrylonitrile copolymer (SAN) through a boundary layer of SAN graft. The dispersed rubbery
phase is rubber polymerized from butadiene. Styrene and acrylonitrile are graft-polymerized to the rubber
thus forming the boundary layer between the dispersed rubber phase and the continuous glassy matrix.
Increased molecular weight of SAN improves product strength and ease of processability, while the
concentration, size, and distribution of the rubber particles influence product toughness and impact
strength. By a careful variation of the parameters controlling the phases, a family of ABS with a broad
range of properties has been developed.
D. CONDENSATION POLYMERS
A large number of commercially important condensation polymers are employed as homopolymers.
These include those polymers that depend on crystallinity for their major applications, such as nylons
and fiber-forming polyesters, and the bulk of such important thermosetting materials like phenolics and
urea–formaldehyde resins. In many applications, condensation polymers are used as copolymers. For
example, fast-setting phenolic adhesives are resorcinol-modified, while melamine has sometimes been
incorporated into the urea–formaldehyde resin structure to enhance its stability. Copolyesters find
application in a fairly broad spectrum of end uses.
The diverse end-use pattern of copolyesters derives from the rather broad structural modifications
that are in general possible with condensation polymers. Addition polymers consist of a chain of carbon
atoms on their backbones. Modification of these polymers is limited essentially to the manipulation of
the pendant group. On the other hand, for condensation polymers, in addition to the modification of the
pendant structure, there is the benefit of the additional possible structural alteration of the backbone. In
the case of polyesters and copolyesters a variety of monomers offers this possibility: aromatic, alicyclic,
and unsaturated diols and dibasic acids. As discussed in Chapter 4, the presence of aromatic or alicyclic
rings on the chain backbone enhances chain stiffness, while the presence of oxygen facilitates chain
rotation and consequently increases chain flexibility. Also, by a suitable choice of monomers, the spacing
between the polar ester groups and hence the cohesive energy density can be controlled. The numerous
and varied applications of copolyesters are a consequence of the literally infinite combinations of dibasic
acids and diols that are possible. Some of these applications include surface coatings, polyurethane
polyester–polyether thermoplastic elastomers, laminating resins, and thermosetting molding compounds.
Most polymers that function properly at ambient temperature quite frequently have limited performance at sustained elevated temperatures. This invariably limits the utility of polymeric materials. The
low thermal stability is generally due to decreased crystallinity and/or thermal decomposition. Polymer
chemists have, through some ingenious ways, synthesized polymer — such as aromatic polyimides and
the so-called ladder polymers — specifically designed for high-temperature applications. However, it
has also been possible to modify polymers to improve their thermal stability and hence extend their
range of utility. A few examples of condensation polymers illustrate this point.
1. Acetal Copolymer
The elemental composition of the backbone of a polymer determines its thermal stability. Most olefin
polymers with carbon–carbon backbone degrade thermally by random chain scission. They are usually
difficult to stabilize thermally through copolymerization. On the other hand, heterochain addition polymers degrade thermally by depolymerization or successive unzipping of monomers. If a comonomer is
incorporated into the polymer backbone to interrupt the unzipping action, improved thermal stability of
the polymer can be achieved. This principle is illustrated by the thermal stabilization of acetal polymers.
Acetal homopolymer or polyoxymethylene is a formaldehyde polymer that is characterized by a good
balance of physical, mechanical, chemical, and electrical properties. However, the polymer has hemiacetal
hydroxy end groups that render the polymer thermally unstable. One way of stabilizing the homopolymer
is by the use of appropriate additives. Another approach is to transform the unstable end groups into
stable end groups by “end capping” through acetylation or methylation. The polymer reaction is carried
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POLYMER SCIENCE AND TECHNOLOGY
Figure 5.2 Diglycidyl ether of bisphenol A (DGEBA).
out in the solid state to produce a polymer that is sufficiently thermally stable to be fabricated by melt
processing.
In contrast to acetal homopolymer, acetal copolymers have built-in heat stabilization. They are
prepared by copolymerization of trioxane with small amounts of comonomer, usually cyclic ethers like
ethylene oxide or 1,3-diozolane.
(CH2O)3 +
CH2
CH2
CH2O
CH2O
CH2O
CH2CH2O
(Str. 1)
O
This results in the random distribution of C–C bonds in the polymer chain. The depolymerization of
ethylene oxide units is much more difficult than that of oxymethylene units. Thus copolymerization
confers thermal stability on the acetal copolymer. The copolymer exhibits good property retention when
exposed to hot air at temperatures up to 220°F or water as hot as 180°F for long periods of time. For
intermittent use, higher temperatures can be tolerated.
2. Epoxies
Epoxies are polymer materials characterized by the presence of reactive terminal epoxide groups. A
frequently used epoxy resin is diglycidyl ether of bisphenol A (DGEBA) (Figure 5.2). Epoxy resins,
because of their versatility, are used in a variety of applications in protective coatings, adhesives,
laminates, and reinforced plastics and in electrical and electronic devices. Epoxy resins, however, exhibit
lower heat resistance than phenolics due to the lower aromatic units in their structures. A combination
of the desirable characteristics of phenolics and epoxies is obtained in epoxy–novolak, which is a type
of multifunctional epoxy resin based on the modification of epoxy resin with novolak phenolics
(Figure 5.3). In this system, the phenolic component confers thermal stability, while the epoxide group
provides sites for cross-linking. The strength–temperature profiles for three unmodified epoxies are
shown in Figure 5.4A. It can be observed that none of these adhesives shows any appreciable strength
retention beyond 100°C. However, the phenolic-modified epoxy adhesive system exhibits good strength
retention even up to 300°C (Figure 5.4B).
3. Urea–Formaldehyde (UF) Resins
Another example of the improvement of properties of condensation polymers through copolymerization
is illustrated by recent work with urea–formaldehyde (UF) resins.3,4 The preponderance of wood adhesive
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POLYMER MODIFICATION
CH2
CH2
O
CH2
O
O
CH
CH
CH
CH2
CH2
CH2
O
O
O
CH2
CH2
CH2
Figure 5.3 Structure of novolak-modified epoxy resin.
Figure 5.4 Comparison of temperature dependence of adhesive strength. (A) Three Araldite epoxy adhesives;
(B) Epoxy-phenolic copolymer. (From Wake, W.C., Adhesion and the Formulation of Adhesives, Applied Science
Publishers, London, 1976.)
bonding is done with urea–formaldehyde and phenol–formaldehyde (PF) adhesives. Compared to PF resins,
bonding with UF resins is cheaper and can be carried out under a wider variety of conditions. However,
PF-bonded wood products can be employed in outdoor structural applications, while the poor stability of
UF-bounded wood products limits their use to interior, nonstructural applications. Evidence of these
limitations includes strength losses of UF-bonded solid wood joints, irreversible swelling of UF-bonded
composite panels, and formaldehyde release. Two bond degradation processes viewed currently as responsible for the poor durability of UF are hydrolytic scission and stress rupture. Some of the molecular structural
factors that contribute to these processes are (1) low and nonuniform distribution of cross-links in cured
UF resins, which lead to extreme sensitivity to small losses in cross-link density arising from hydrolytic
scission or stress rupture, and (2) the brittleness of the cured resin due partly to the inherent rotational
stiffness of urea molecules leading to inability to respond reversibly to stresses arising from cure shrinkage
and moisture-related wood swelling and shrinking. To minimize these defects, urea derivatives of flexible
di- and trifunctional amines were incorporated into UF resin structure through copolymerization. Figure 5.5
shows the improved response to cyclic wet–dry stress of joints bonded with a modified UF resin compared
with joints bonded with unmodified resin. The amine modifier employed in this case was the urea derivative
of propylene oxide-based triamine with the structure shown in Figure 5.6.
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POLYMER SCIENCE AND TECHNOLOGY
Figure 5.5 Enhanced soak–dry stress resistance of solid wood joints bonded with UF-resin modified with the
urea derivative of a flexible propylene oxide-based triamine.
CH2
CH3
CH2
C
CH2
O
CH2
CH2CH(CH3)
O
O
x
NH2
CH2CH(CH3)
CH2CH(CH3)
z
y
NH2
NH2
(x + y + z = 5.3)
Figure 5.6 Structure of propylene oxide-based triamine modifier.
Example 5.3: Internal stresses are developed as a result of shrinkage during adhesive cure. These stresses
can be determined by measuring the deflection of an aluminum strip coated on one side with a thin film
of the adhesive (Figure E5.3). The maximum in the curve for the unmodified resin was found to coincide
with the fragmentation of the adhesive film. Explain the observed deflections under ambient cure of two
such aluminum strips coated, respectively, with a modified and an unmodified UF resin. The modifier
is the urea derivative of propylene oxide-based triamine shown in Figure 5.6.
Figure E5.3 Relative internal stresses generated during cure by unmodified and modified UF adhesives.
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POLYMER MODIFICATION
Solution: As a result of the intrinsic rotational stiffness of the urea molecule, UF resins develop a
considerable amount of internal stresses during cure. These stresses are seen to build up with time under
ambient cure. However, for the unmodified resin after 60 days of ambient cure, the magnitude of the
stresses resulted in a breakup of the resin. This relieved the internal stress, and consequently the deflection
of the aluminum strip began to decline. On the other hand, with the modified resin, the flexibility of the
modifying agent incorporated into the resin cure. Thus the aluminum strip coated with the modified
resin structure provides an internal mechanism for relieving some of the stresses generated during resin
showed much smaller deflection for the same duration of cure. Apparently even after 100 days of ambient
cure, the internal stress generated in the modified resin had not reached a level where it became greater
than the cohesive strength of the cured resin.
III.
POSTPOLYMERIZATION REACTIONS
As indicated earlier, another powerful tool for upgrading polymer properties is the postpolymerization
reaction of preformed polymers. These reactions may occur on reactive sites dispersed in the polymer
main chain. Such reactions include chain extensions, cross-linking, and graft and block copolymer
formation. The reactions may also occur on reactive sites attached directly or via other groups/chains
to the polymer backbone. Reactions of this type are halogenation, sulfonation, hydrolysis, epoxidation,
surface, and other miscellaneous reactions of polymers. In both cases these types of reactions transform
existing polymers into those with new and/or improved properties.
A. REACTIONS OF POLYSACCHARIDES
Even though the treatment of polysaccharide chemistry falls outside the scope of this volume, reactions
of the polysaccharide cellulose provide an excellent illustration of how the modification of a polymer,
in this case a natural polymer, can transform an otherwise intractable material into one that can be readily
fabricated.
1. Cellulose Derivatives
Cellulose is a polysaccharide — a natural polymer composed of glucosidic rings linked through oxygen
bridges (Figure 5.7). The repeating unit has three hydroxyl groups and an acetal linkage. The cyclic
nature of the repeating units results in stiff molecules. In addition, the β-(1→4) links between the
anhydro-D-glucose units confer linearity on cellulose molecules. These factors coupled with the strong
hydrogen-bond formation through the hydroxyl groups make cellulose a highly crystalline polymer. As
a result of the extremely high interchain bonding between cellulose molecules, cellulose is insoluble and
infusible. It burns rather than melts. To make cellulose processable it is necessary to reduce its melting
point below its decomposition temperature. This is achieved by derivatization, which essentially breaks
down the intrinsic hydrogen bonding while also interfering with the crystalline nature of the polymer.
In the preparation of cellulose derivatives a controlled degree of substitution of the three hydroxyls
is necessary. Complete reaction of the three hydroxyls is generally not desirable. In addition, the fine
structure of cellulose dictates both the course of reaction and the properties of the end products. When
reacting with cellulose, reagents normally attack the most accessible (noncrystalline) regions first.
Therefore, if the reaction is stopped prematurely, the reacted groups will be concentrated in certain
regions rather than distributed randomly within the cellulose structure. For example, 30% acetylated
cellulose prepared by direct acetylation has different properties from that prepared by 70% hydrolysis
of the acetyl groups in cellulose triacetate.
The most important cellulose derivatives are cellulose esters and ethers. Cellulose esters are prepared
by the reaction of an activated cellulose with the corresponding carboxylic acid, acid anhydride, or acid
halide. Esterification is taken to completion (triester) and then hydrolyzed back to the desired free
hydroxyl content. Viscosity is controlled by holding the reaction at the acid stage until the molecular
weight has been reduced to the desired level. For plastics, a relatively high molecular weight is desirable,
while for applications as adhesives, lacquer, and hot melts, a much lower molecular weight is more
suitable.
The history of the development of polymers is intimately tied to cellulose nitrate, also referred to as
nitrocellulose. This inorganic cellulose ester is obtained by reacting cellulose with a mixture of nitric
acid and sulfuric acid for about 30 min.
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POLYMER SCIENCE AND TECHNOLOGY
HO
H
H
OH
OH
H
H
O
CH2OH
H
H
O
CH2OH
O
H
OH H
H
OH
O
H
H
n-2
H
OH
OH
H
H
O
CH2OH
H
OH
Figure 5.7 Structure of cellulose.
→ Cell ±O ±NO + H O
Cell − OH + HNO3 ←
2
2
(Str. 2)
Some degradation accompanies the nitration process. Products with different nitrate contents are
obtained. Fully nitrated products have a nitrogen content of about 14.8%. The properties and applications
of the nitrated material depend on its degree of nitration; plastics and lacquer-grade material contain
10.5 to 12% nitrogen, which corresponds to the dinitrate roughly. Products with nitrogen content greater
than 12.5% are used for explosives.
Cellulose ethers like methyl cellulose are prepared by reacting sodium cellulose (obtained by the
action of aqueous sodium hydroxide on cellulose) with the corresponding alkyl halide.
CellOH + RCl + NaOH → Cell − O − R + NaCl + H 2O
(Str. 3)
The ether content and the viscosity of the product are controlled by controlling the reaction conditions.
In some cases, cellulose ethers are made by reacting sodium cellulose with ethylene or propylene oxide
to yield the corresponding hydroxyalkyl ether. Such hydroxy-terminated cellulose ethers are of considerable interest.
Ethyl cellulose is the most important of the cellulose ethers. Commercial ethyl cellulose, which has
about 2.4 to 2.5 ethoxy groups per glucose residue is a molding material that is heat stable and has low
flammability and high impact strength. It is flexible and tough even at low temperatures; it has a relatively
high water absorption, which is, however, lower than that of cellulose acetate.
2. Starch and Dextrins
Like cellulose, starch is also a polysaccharide that on complete hydrolysis yields glucose units. However,
there are two significant differences between starch and cellulose. Unlike those in cellulose, the anhydroD-glucose units in starch are linked through α-(1 → 4) glycosidic bonds. Also, in contrast to the completely linear structure of cellulose molecules, the structure of starch is a mixture of linear amylose
molecules and branched amylopectin chains. Granular starch has no adhesive use. Therefore for adhesive
and most applications, starch must be dispersed in solution by cooking in water until the granules swell
and rupture. Such aqueous dispersions of starch are extremely viscous. In order to reduce the viscosity
of such starch dispersions, starch must be modified through processes that involve partial depolymerization and/or rearrangement of molecules. To effect this conversion, starch must be subjected to
hydrolytic, oxidative, or thermal processes, which yield three classes of products: acid-converted (thin
boiling) starches; oxidized starches; and dextrins.
Conversion processes cause weakening and/or solubilization of starch granules, resulting in products
that are more readily dispersible and of lower viscosities. Thin boiling starches are produced by reactions
of granular starch with warm mineral acids such as HCl or H2SO4. Oxidized starches are made by the
reaction of starch with sodium hypochlorite. These reactions introduce carbonyl and carboxyl groups
and cleave the glycosidic linkages. Dextrins are degradation products of starch produced by heating
starch in the presence or absence of hydrolytic agents. Depending on the conditions of conversion, three
types of dextrins are produced: white dextrins, yellow (canary) dextrins, and British gums. The conversion
mechanism is complex, but is thought to involve hydrolytic breakdown of starch molecules into smaller
fragments followed by their rearrangement/repolymerization into a branched polymer structure
(Figure 5.8).
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POLYMER MODIFICATION
Figure 5.8 Hydrolysis and repolymerization during dextrination of starch. (From Jarowenko, W., Handbook of
Adhesives, 2nd ed., Skeist, I., Ed., Van Nostrand Reinhold, New York, 1977. With permission.)
Example 5.4: Explain the following observations:
a. Cellulose acetate-molded and extruded articles are used extensively where extreme moisture
resistance is not required. On the other hand, plastic sheets and films made from cellulose
propionate, cellulose butyrate, or cellulose-acetate-butyrate are hydrophobic.
b. Products from cellulose ethers with the same degree of substitution as cellulose ester have similar
properties but are more resistant to hydrolysis.
Solution:
a. Cellulose acetate is an ester formed by the reaction of acetic acid with the polyhydric alcohol
cellulose. Esterification reactions are typically equilibrium reactions.
→ Cell − O − CO − R + H O
Cell − OH + R − COOH ←
2
(Str. 4)
In the presence of water (moisture) the reverse reaction can occur and is indeed favored at ordinary
conditions for organic acids. Consequently, cellulose esters from organic acids like acetic acid
and higher homolog are prepared only by the removal of water as it is formed. The resulting
product is moisture sensitive, the degree of which decreases with the progressive hydrocarbon
nature of R. Therefore while cellulose acetate (R = –CH3) is susceptible to hydrolysis, cellulose
propionate (R = –CH2–CH2–CH3) and cellulose butyrate [R = –(CH2)3–CH3] are hydrophobic.
b. The formation of cellulose ethers, unlike that of cellulose esters, is not reversible. Cellulose
ethers are therefore less sensitive to hydrolysis than cellulose esters.
Cell − OH + RCl + NaOH → Cell − O − R + NaCl + H 2O
(Str. 5)
B. CROSS-LINKING
In Chapter 2 we indicated that the formation of a polymer requires that the functionality of the reacting
monomer(s) must be at least 2. Where the functionality of one of the monomers is greater than 2, then
a cross-linked polymer is formed. Thermosets like phenol–formaldehyde, urea–formaldehyde, and epoxy
resins develop their characteristic properties through cross-linking. In this section our discussion is
confined to those polymeric systems designed with latent cross-linkability that under appropriate conditions can be activated to produce a polymer with desirable properties.
1. Unsaturated Polyesters
Unsaturated polyesters represent an excellent illustration of a polymeric system with a latent ability to
cross-link. Unsaturated polyesters are, as the name implies, unsaturated polyester prepolymers mixed with
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POLYMER SCIENCE AND TECHNOLOGY
cross-linking monomers and catalysts. The resulting mixture is normally a viscous liquid that can be poured,
sprayed, or shaped into the desired form and then transformed into a thermosetting solid by cross-linking.
The unsaturated polyester prepolymers are obtained from the condensation of polyhydric alcohols and
dibasic acids. The dibasic acid consists of one or more saturated acid and/or unsaturated acid. The saturated
acid may be phthalic anhydride, adipic acid, or isophthalic acid, while the unsaturated acid is usually
maleic anhydride or fumaric acid. The polyhydric alcohols in common use include glycol (such as ethylene
glycol, propylene glycol, diethylene glycol), glycerol, sorbitol, and pentaerythritol (Equation 5.1).
(5.1)
The reactive monomer responsible for the cross-linking reaction is normally styrene, vinyl toluene,
methyl methacrylate, or diallyl phthalate. The unsaturated polyester prepolymers are, by themselves,
relatively stable at ambient temperatures. When they are added to the cross-linking monomers, however,
the blend is extremely unstable. To prevent premature gelation and to control the cross-linking process,
inhibitors like hydroquinone are added to the blend. The cross-linking process is usually initiated by an
organic peroxide added by the end user. The peroxide may be activated by heat. For room-temperature
cure, accelerators and activators such as cobalt salts are also added. The cross-linking reaction can be
represented as follows (Equation 5.2):
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POLYMER MODIFICATION
O
O
C
O
CH
CH
C
O
C
+
O
C
CH2
O
CH
CH
X
C
O
O
O
O
O
CH
C
CH2
CH
C
CHX
O
CH
CH
C
CH2
O
CH
C
O
O
O
CH
CH
X
CH
C
O
(5.2)
O
O
O
Alkyd resins constitute a special class of unsaturated polyesters. They are used predominantly as
surface coatings (organic paints) and, to a limited extent, as molding compounds. Unlike the related
polyester molding compounds (used in the manufacture of automotive parts like distributor caps, ignition
coil towers, and similar electrical parts, the cross-linking monomer in alkyd molding compounds is
nonvolatile, e.g., allyl phthalate. The organic peroxide catalyst decomposes on the application of heat
and initiates cross-linking yielding on infusible, insoluble solids.
Alkyd resins used as surface coatings are usually modified by the addition of fatty acids derived from
mineral and vegetable oils. Those modified with unsaturated acids such as oleic acid (I) or linoleic acid
(II) are the air-drying type cured through oxidation by atmospheric oxygen. Those types of surface
coatings that dry through chemical reactions are known as varnishes. Others that dry through solvent
evaporation are referred to as lacquers.
CH3 − (CH 2 )7 − CH = CH − (CH 2 )7 − COOH
oleic acid (I)
CH3 − (CH 2 )7 − CH − CH − CH − CH − (CH 2 )5 − COOH
linoleic acid (II)
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(Str. 6)
(Str. 7)
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POLYMER SCIENCE AND TECHNOLOGY
Example 5.5: An unsaturated polyester based entirely on maleic anhydride as the dibasic acid, when
cross-linked with styrene, forms a hard and rigid polymer. However, that based on maleic anhydride
mixed with some phthalic anhydride forms a less rigid polymer. Explain.
Solution: As we shall see in Chapter 8, the copolymerization of maleic anhydride and styrene shows
a strong tendency toward alternation (r1 r2 = 0.06). Consequently, a maleic anhydride-based unsaturated
polyester cross-linked with styrene forms a large number of short cross-links, resulting in a rigid polymer.
With the addition of phthalic anhydride, the tendency toward alternation is reduced due to the occasional
intervention of phthalic anhydride. The reduction of cross-link density results in a less rigid polymer.
2. Vulcanization
Elastomers such as 1,4-diene polymers — polychloroprene, polybutadiene, and polyisoprene — and
copolymers — butadiene–acrylonitrile and butadiene–styrene — must possess the capability of undergoing large deformations under stress and the essential property of rapid and complete recovery from
such deformations on the removal of the imposed stress. This means, in molecular terms, that the longrange movements of the polymer molecules must be restrained at the same time that high local segmental
mobility is permitted. To acquire this property, an elastomer needs to be lightly cross-linked. The process
by which such a network of cross-links is introduced into an elastomer is called vulcanization. It
transforms an elastomer from a weak thermoplastic mass without useful properties into a strong, elastic,
tough rubber. Vulcanization decreases the flow of an elastomer and increases its tensile strength and
modulus, but preserves its extensibility. Vulcanization, discovered by Goodyear in 1939, achieved by
heating the elastomer solely with sulfur is a slow and inefficient process. It can be speeded up considerably
and the sulfur waste reduced substantially by the addition of small amounts of organic and inorganic
compounds known as accelerators. Many accelerators themselves require the presence of activators or
promoters to function optimally. Some accelerators used include sulfur-containing compounds and a few
nonsulfur-containing compounds, as shown in Table 5.4.
Activators are normally metallic oxides such as zinc oxide. They function best in the presence of
soap such as metallic salt of stearic acid. The use of accelerators and activators has increased the efficiency
of cross-linking in some cases to less than two sulfur atoms per cross-link.
a. Rubbers33
The early synthetic rubbers were diene polymers such as polybutadiene. Diene elastomers possess a
considerable degree of unsaturation, some of which provide the sites required for the light amount of
cross-linking structurally necessary for elastomeric properties. The residual double bonds make diene
elastomers vulnerable to oxidative and ozone attack. To overcome this problem, saturated elastomers
like butyl rubber and ethylene–propylene rubber (EPR) were developed. These rubbers were, unfortunately, not readily vulcanized by conventional means. To enhance cure, it was therefore necessary to
Table 5.4 Some Acclerators Used in Vulcanization.
Accelerator
2-mercaptobenzothiazole
Tetramethylthioureadisulfide
Diphenylguanidine
Zinc butyl xanthate
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Structure
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POLYMER MODIFICATION
CH3
C
CH3
CH2
CH3
C
CH2
CH3
n
Polyisobutylene
CH2
CH3
CH2
C
CH
CH2
m
n
x
Isobutylene-isoprene copolymer
CH2
CH2
m
CH
CH3
Ethylene-propylene Copolymer
(EPR)
CH2
n
x
CH2
CH2
m
CH
CH3
CH
n
CH
CH
CH2 CH
CH2 C
o
x
CH
CH3
Ethylene-propylene-diene terpolymer
Figure 5.9 Creation of vulcanization sites for butyl and ethylene–propylene rubbers through copolymerization.
incorporate some unsaturation into the structure of these polymers through copolymerization of unsaturated monomers. The usual monomers for butyl rubber and EPR are shown in Figure 5.9.
The comonomer shown for EPR is dicyclopentadiene. Other comonomers are ethylidene norbornene
(obtained from the reaction of butadiene with cyclopentadiene) and 1,4-hexadiene. Each of the diene
copolymers with ethylene and propylene produces different characteristics in the final elastomer. Dicyclopentadiene yields a branched polymer with slow cure rates because of the low reactivity of the second
double bond. With ethylidene norbornene, both linear and branched polymers can be produced and sulfur
vulcanization is improved. The polymer resulting from the incorporation of 1,4-hexadiene into EPR
possesses good processing characteristics and heat resistance. Since EPR is considered saturated, it
usually requires peroxide cure, while EPDM polymers are cured normally by conventional sulfur
vulcanization. The type of cure is dictated by the polymer and the expected processing technique
(extrusion, calendering, molding, etc.). The commercial cross-linking of diene polymers is carried out
almost exclusively by heating with sulfur (Equation 5.3).
(5.3)
b. Polyolefins and Polysiloxanes
While the first vulcanization involved heating the elastomer with sulfur, it has since been recognized that
neither heat nor sulfur is imperative for the vulcanization process. Vulcanization or cure can be effected
by nonsulfur-containing compounds, including peroxides, nitro compounds, quinones, or azo compounds.
Polyethylene, ethylene–propylene copolymers, and polysiloxanes are cross-linked by compounding them
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POLYMER SCIENCE AND TECHNOLOGY
with a peroxide and heating them. The process involves the formation of polymer radicals followed by
the coupling of these radicals (Equations 5.4–5.6).
(5.4)
(5.5)
(5.6)
The efficiency of this process is usually less than one cross-link per peroxide molecule decomposed. To
increase the cross-linking efficiency, small amounts of unsaturation are introduced into the polymer
structure. We have already discussed EPDM polymers, which are essentially diene monomers copolymerized with ethylene–propylene (EPR) polymers. For polysiloxanes, copolymerization of small
amounts of vinyl–methylsilanol greatly enhances cross-linkability (Equation 5.7). The unsaturation introduced into an otherwise saturated structure provides additional sites for cross-linking through chain
reaction.
CH2
CH2
CH
HO
CH3
Si
OH
+
HO
CH3
Si
CH
OH
O
CH3
CH3
Si
O
CH3
Si
(5.7)
O
CH3
C. HYDROLYSIS11
A primary use of poly(vinyl acetate) is in the production of water-based emulsion paints in addition to
its wide use in emulsion-type and hot-melt adhesives. It is also used in the production of poly(vinyl
alcohol) and poly(vinyl acetal) polymers, which cannot be prepared directly since their monomers are
unknown.
Poly(vinyl alcohol) (PVA) is prepared by the hydrolysis (or more correctly alcoholysis) of poly(vinyl
acetate) with methanol or ethanol. The reaction is catalyzed by both bases and acids, but base catalysis
is normally employed because it is faster and free of side reactions (Equation 5.8).
O
CH2
CH
+
n
O
C
nCH3OH
H+ or OH-
CH2
CH
OH
+
nCH3 C
OCH3
(5.8)
n
O
CH3
Because of its water solubility, poly(vinyl alcohol) is used as a thickening agent for various emulsion
and suspension systems. With its high hydroxyl content, PVA is used widely as water-soluble adhesive
with excellent binding capacity for cellulosic materials like paper.
Partially hydrolyzed poly(vinyl acetate) contains both hydroxyl and acetate groups. When the OH
groups in partially hydrolyzed poly(vinyl acetate) are condensed with aldehydes, acetal units are formed.
The resulting polymer contains acetal, hydroxyl, and acetate groups and is known as the poly(vinylacetals) (Equation 5.9).
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POLYMER MODIFICATION
CH2
CH
CH2
OH
CH
CH2
OH
CH
CH2
OH
CH
RCHO
O
C
O
CH3
CH2
CH
CH2
O
CH
O
CH2
CH
OH
O
O
CH3
H
acetal
(5.9)
CH
C
C
R
CH2
alcohol
acetate
The reaction of butyraldehyde or formaldehyde results in poly(vinyl butyral) or poly(vinyl formal),
respectively. The most important of the poly(vinylacetals) is by far poly(vinyl butyral). The residual OH
groups in poly(vinyl acetal) can condense with methylol groups in PF, MF, and UF resins. For example,
poly(vinyl formal) cured with phenolic resins was first used as a structural adhesive for metals. Poly(vinyl
butyral) finds important application as the interlayer in laminated automotive and aircraft safety glass.
Laminated glass, also made from poly(vinyl butyral), is used in buildings for controlled transmission of
light and heat through reduction of glare, heat loss, and UV, thus providing aesthetic appeal. In these
applications, the residual hydroxyl groups provide the needed strength and adhesion to glass.
D. BLOCK AND GRAFT COPOLYMER FORMATION6,7
Block and graft copolymer formation involves the reaction of a previously formed homopolymer or
copolymer with fresh monomers. Consequently, normal methods of polymerization can be utilized. For
vinyl monomers two general methods are employed: (1) the polymerization of a monomer in the presence
of a polymer by the initiation of growth through chain transfer; and polymerization of a monomer in
the presence of a polymer containing reactive sites. The first procedure necessarily leads to the formation
of graft copolymers. In the second case, block copolymers are formed from polymers with terminal
active sites while graft copolymerization occurs on active sites located on the polymer backbone or
pendant groups. In both of these methods, the polymerization of a monomer in the presence of a polymer
results in a mixture of products: (1) the initial homopolymer that did not participate in the reaction;
(2) the homopolymer of the fresh monomer; (3) the cross-linked parent homopolymer through graft
polymerization or the branched block copolymer; and (4) the desired copolymer. The composition of
the product mixture will depend on the nature of the polymer, monomer, and initiator. For example, the
polymerization of methyl methacrylate in the presence of polystyrene yields appreciable amounts of
graft copolymer with benzoyl peroxide as the initiator. However, if AIBN is the initiator, the amount of
graft copolymer formed is considerably smaller. In contrast, the polymerization of vinyl acetate in the
presence of poly(methyl methacrylate) yields appreciable amounts of the graft copolymer irrespective
of the nature of the initiator. In any case, while it is possible in principle to obtain a pure product by
employing general separation processes, this is often problematic for industrial purposes.
1. Block Copolymerization8,9
The preparation of block copolymers requires the presence of terminal reactive groups. A variety of
techniques have been used to achieve this. We briefly discuss a few of them.
Block copolymers of butyl acrylate–styrene and acrylonitrile–styrene have been prepared by irradiating butyl acrylate or acrylonitrile containing a photosensitive initiator (e.g., 1-azo-bis.1-cyanocyclohexane) with an intensive UV radiation. This creates a radical-rich monomer that when mixed with
styrene yields the appropriate block copolymer.
In situ block polymer formation is obtained by using a mixture of water-soluble and oil-soluble
monomers. An example involves dissolution of acrylic acid or methacrylic acid in water followed by
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POLYMER SCIENCE AND TECHNOLOGY
Table 5.5 Various Block Copolymers Prepared
by Mechanical Rupture of Polymer Chains
in Presence of Monomer
Polymer Unit
Monomer
Acrylamide
Isobutylene
Methyl acrylate
Methacrylonitrile
Methacrylonitrile-co-vinyl chloride
Methyl methacrylate
Methyl methacrylate-co-methacrylonitrile
Styrene
Vinyl acetate
Vinyl chloride
Acrylonitrile
Acrylonitrile
Styrene
Vinylidene chloride
Vinyl chloride
Vinyllidene chloride
Acrylonitrile
Methyl methacrylate
Methacrylonitrile
Styrene
Vinylidene chloride
Styrene
Methyl methacrylate
Vinylidene chloride
Vinyl chloride
Methyl vinyl ketone
From Fetters, E.M., Ed., Chemical Reactions of Polymers, Interscience, New York, 1964. With permission.
styrene emulsification in the water. With persulfate initiator, polymerization of acid occurs in the aqueous
phase until the growing chains diffuse into the micelle containing styrene monomer, whereupon styrene
chain growth ensues resulting in the formation of acrylic/methacrylic acid–styrene block copolymer.
When a polymer chain is ruptured mechanically, terminal-free radicals can be generated, and these
can be utilized to initiate block copolymerization. Under an inert atmosphere, block copolymers can be
produced by cold milling, or mastication of two different polymers or of a polymer in the presence of
a second monomer. This generally results in the formation of graft copolymers in addition to the block
copolymers since radicals can be located in nonterminal positions by chain transfer. However, predominant yield of block copolymers is obtained by milling monomer-swollen polymers. The success of this
technique depends on the physical state of the polymer. Generation of radical is favored if the polymer
exists at or near the glassy state; otherwise, polymer flow rather than bond rupture will occur. Table 5.5
shows some block copolymers prepared by this technique.
When polymer chains are terminated by labile end groups, block polymers can be produced by
irradiation. An example is the irradiation of terminal-brominated styrene dissolved in methyl methacrylate
to yield styrene–methyl methacrylate block copolymer (Equation 5.10).
CH3
CH2
CH
Br
hν
MMA
CH2
CH
CH2
C
C
(5.10)
O
O
n
n
CH3
m
A common technique of preparing block copolymers is the introduction of peroxide groups into the
polymer backbone or as stable end groups. The polymers are then mixed with fresh monomer, and the
peroxide groups are decomposed under appropriate conditions to yield block copolymers. For example,
polymeric phthaloyl peroxide was polymerized to a limited extent with styrene. The resulting polymer
was mixed with methyl methacrylate. On decomposition, the internal and terminal peroxide groups
formed radicals that initiated the polymerization of methyl methacrylate, as shown below in
Equation 5.11.
Copyright 2000 by CRC Press LLC
135
POLYMER MODIFICATION
O
O
O
O
C
C
O
O
O
O
C
C
O
CH2
CH
n
O
O
O
O
C
C
O
CH2
heat
MMA
CH
(5.11)
m
CH3
CH2
C
C
O
O
O
C
C
O
CH2
CH
O
O
m
CH3
p
2. Graft Copolymerization8
There are essentially three approaches to the preparation of graft copolymers via free radical mechanism:
1. Chain transfer to a saturated or unsaturated polymer
2. Activation by photochemical or radiative methods
3. Introduction and subsequent activation of peroxide and hydroperoxide groups
Graft copolymer formation by chain transfer requires three components: a polymerizable monomer,
a polymer, and an initiator. By definition, the initiator serves to create active sites either on the polymer
or on the monomer. The amount of initiator added to a polymerization system is generally low. Therefore,
if this is the sole source for the initiation of radical sites on the backbone of the polymer by way of
chain transfer, only a relatively few branches will be produced. Consequently, to achieve a reasonable
amount of grafting, the polymer itself must be capable of generating radicals through hydrogen abstraction and hence promote chain transfer. As we shall see in Chapter 8, the relative reactivity ratios dictate
that for copolymerization to occur the nature of the monomer must be such that it can react with the
polymer radical. This means that not all monomers can be grafted onto a polymer backbone, and, by
the same argument, not all polymers can react with a given monomer. For example, whereas vinyl acetate
can be grafted onto poly(vinyl chloride) or polyacrylonitrile, hardly any grafting occurs in the case of
vinyl acetate or vinyl chloride and polystyrene; styrene or vinyl chloride and poly(vinyl acetate); or
styrene and poly(vinyl chloride).
Sites that are susceptible to chain transfer may be introduced into the polymer structure during
synthesis or as a postreaction. An example of the latter case involves the introduction of a reactive
mercaptan group by the reaction of thioglycolic acid or hydrogen sulfide with a copolymer of methyl
methacrylate and glycidyl methacrylate (Equation 5.12).
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136
POLYMER SCIENCE AND TECHNOLOGY
CH3
CH2
CH3
C
CH2
C
C
O
C
CH3
HS
CH2
COOH
CH2
O
CH3
C
CH2
C
C
O
C
O
O
O
O
CH3
CH2
CH3
CH2
CH
HS
CH2
COO
CH
HS
CH2
COO
CH2
O
CH2
(5.12)
O
Monomers such as styrene, acrylate, and methacrylate have been grafted onto this polymer, with a
relatively high yield of pure graft copolymer. A commercially important graft copolymer resulting from
transfer to unsaturated polymer is impact polystyrene, which is butadiene grafted to polystyrene. The
nature of the transfer process in this case results in a copolymer with the rubber (polybutadiene) graft
existing as a dispersed phase within a styrene matrix. Such a copolymer morphological structure reduces
the brittleness of polystyrene, and this enhances its impact resistance.
Active sites can be generated on a polymer to initiate graft copolymerization by ultraviolet light and
high-energy irradiation. The process may involve (1) simultaneous irradiation of polymer in contact with
monomer; (2) irradiation of polymer predipped in the monomer; (3) pre-irradiation of the polymer in
the absence of air (O2) followed by exposure to the monomer; and (4) pre-irradiation of the polymer in
air to form peroxide and subsequent decomposition in the presence of the monomer. The first two
techniques are known as mutual irradiation, as distinct from pre-irradiation.
Photolytic activation of polymer growth is based on the ability of certain functional groups to absorb
radiation. This generally involves polymers with pendant halogen atoms or carbonyl groups, which are
highly susceptible to activation by UV radiation. When sufficient energy is absorbed, bond scission occurs
resulting in the generation of radicals. Monomers such as acrylonitrile, methyl methacrylate, and vinyl
acetate have been grafted onto poly(vinyl methyl ketone) by this technique. Other examples include grafting
of styrene and methyl methacrylate on brominated styrene. Mutual irradiation with UV in the presence of
a photosensitizer has also been used to graft methyl methacrylate and acrylamide to natural rubber.
Generation of active sites for radical graft copolymerization using X-rays or γ-rays is less selective. In
this case, hydrogen cleavage and carbon–carbon bond scission occur resulting in homopolymerization of
the vinyl monomer, cross-linking in addition to the grafting reaction. Table 5.6 shows some graft copolymers
produced by natural irradiation with high-energy radiation and the properties of the resulting graft copolymer.
Unstable compounds such as peroxides and hydroperoxides are usual initiators for vinyl polymerization reactions. These groups can be introduced into a solid polymer by pre-irradiation in the presence
of air or oxygen. Thermal decomposition of these groups in the presence of monomer yields graft
copolymers. Some examples include grafting of acrylonitrile, styrene, and methyl methacrylate to
polyethylene and polypropylene.
Table 5.6 Properties of Graft Copolymers Produced by Mutual Irradiation of Monomer
Polymer
Polyethylene
Monomer
Acrylonitrile
Vinyl chloride
Acrylate
methacrylate
Polytetrafluoroethylene
Polychlorotrifluoroethylene
Copyright 2000 by CRC Press LLC
Styrene
Styrene
Property Changes
Increased solvent resistance and softening temperature; excellent adhesion
to polar materials
Retention of superior electrical properties while increasing softening point
to 215°C
Following hydrolysis, surface becomes permanently conductive; prevention
of accumulation of static charges; good adhesion to materials like
cellulose, glass, and metals
Increased melt viscosity
Enhanced adhesion, enhanced elimination of plastic flow, and increased
ultimate strength
137
POLYMER MODIFICATION
n HOOC
Ax
COOH
CO
+
n H2N
Ax
By
O
H
C
N
NH2
H
By
N
n
I
n H2N
Ax
NH2
NH
+
2ClCO
Ax
R
COCl
H
O
N
C
R
+
n H2N
O
H
C
N
II
By
By
NH2
H
O
N
C
O
R
C
n
Figure 5.10 Formation of block copolymers from condensation polymers: (I) from two prepolymers with mutually
reactive terminal end groups; (II) by the use of a coupling agent.
So far, our discussion has been restricted to chain block and graft copolymerization. This is largely
because the practical utility of copolymerization is more elaborate in chain polymerization than step
polymerization. Also, in step copolymerization, block copolymers are generally preferred to the other
types of copolymers. Therefore only block step-polymerization copolymers are discussed here and only
in a very limited scope to illustrate the principles involved in their preparation.
Block copolymers have been prepared from condensation polymers by coupling low-molecular-weight
homopolymer with appropriate reactive end groups. These prepolymers may possess terminal end groups
that are different but mutually reactive. If, however, the two polymer blocks have the same or similar
terminal groups, a bifunctional coupling agent capable of reacting with such groups is employed to
prepare the block copolymers. Step-polymerization block copolymer can also be prepared via interchange
reactions. Figure 5.10 illustrates these general principles.
There are a variety of commercially important condensation block copolymers. For example,
alkyd–amine resins are cross-linked block copolymers resulting from the reaction of hydroxyl and
carboxyl groups on alkyd resins with the methylol groups of urea- or melamine-(amine)-formaldehyde
resins. Epoxy resins contain secondary hydroxy and terminal epoxy groups that are capable of reacting
with other functionalities to produce block copolymers. Reaction with the amine groups (primary and
secondary) in polyamide yields block copolymers ranging from rubbery and resilient to hard, tough, and
shock-resistant solids. Epoxy resins can also react with polysulfide liquid polymers to give products
varying in property from brittle to highly flexible. Formaldehyde resins (phenol–formaldehyde,
urea–formaldehyde, and melamine–formaldehyde) react with epoxy resins to yield block copolymers
used in coatings and adhesives.
E. SURFACE MODIFICATION10
Surface reactions of polymers, in strict terms, should include polymer interactions with its environment,
referred to as aging or weathering. The discussion here, however, focuses on the deliberate chemical
modifications of surface properties. Surface reactions, by definition, do not include bulk homogeneous
reactions of polymers, but refer to those confined to the surface and those that generally do not
significantly alter the physical properties of the substrate. Surface reactions have been used commercially
to improve feel, washability, dye retention, antistatic properties, and abrasion resistance of fibers and to
enhance printability, solvent resistance, adhesion, and permeability to liquids and vapors of films. Surface
modification of polymers can be effected by three general procedures: interpolymerization by energy
irradiation or chemical means and by conventional chemical reactions of polymers. Interpolymerization,
as has been discussed, results in the formation of graft or block copolymers. The chemical treatment of
Copyright 2000 by CRC Press LLC
138
POLYMER SCIENCE AND TECHNOLOGY
polymers, including such reactions as oxidation, halogenation, and other standard chemical reactions,
are briefly reviewed.
Surface oxidation reactions have been carried out on a number of polymers, particularly polyethylene.
Surface oxidation techniques include the use of corona discharge, ozone, hydrogen peroxide, nitrous
acid, alkaline hypochloride, UV irradiation, oxidizing flame, and chromic acid. The reactions lead initially
to the formation of hydroperoxides, which catalyze the formation of aldehydes and ketones and, finally,
acids and esters. Surface oxidation treatment has been used to increase the printability of polyethylene
and poly(ethylene terephthalate) and to improve the adhesion of polyethylene and polypropylene to polar
polymers and that of polytetrafluoroethylene to pressure-sensitive tapes. Surface-oxidized polyethylene,
when coated with a thin film of vinylidene chloride, acrylonitrile, and acrylic acid terpolymers becomes
impermeable to oxygen and more resistant to grease, oil, abrasion, and high temperatures. The greasy
feel of polyethylene has also been removed by surface oxidation.
Alkali and acid treatments have also been used to modify surface properties of polymers: sulfonated
polyethylene films treated first with ethylenediamine and then with a terpolymer of vinylidene chloride,
acrylonitrile, and acrylic acid exhibited better clarity and scuff resistance and reduced permeability.
Permanently amber-colored polyethylene containers suitable for storing light-sensitive compounds have
been produced by treating fluorosulfonated polyethylene with alkali. Poly(ethylene terephthalate) dipped
into trichloroacetic/chromic acid mixture has improved adhesion to polyethylene and nylons. Antifogging
lenses have been prepared by exposing polystyrene films to sulfonating conditions. Acid and alkali
surface treatments have also been used to produce desired properties in polymethylmethacrylates,
polyacrylonitrile, styrene–butadiene resins, polyisobutylene, and natural rubber. Surface halogenation of
the diene polymers natural rubber and polyisobutylene resulted in increased adhesion to polar surfaces.
Treatment of a large number of thermoplastics with alkyl or alkenyl halosilanes has yielded materials
with improved heat, stain, and scratch resistance. The use of silane coupling agents in a number of
systems is particularly instructive. These systems include glass- or carbon-fiber-reinforced composites,
phenolic-bonded sand, grinding wheel composites of aluminum oxide, metal-filled resins for the tool
and die industry, etc. In fiber-reinforced composites where the interfacial area between the matrix resin
and fiber reinforcement is very high, adequate fiber–matrix adhesion is necessary for the efficient transfer
of the applied stress across the fiber–matrix interface required for the production of high-performance
composite systems.11 Silane coupling agents are characterized by terminal groups specifically designed
to effect chemical linkage between matrix resin and the fiber reinforcement. A typical example is γaminopropyltriethoxy silane, (C2H5O)3-Si-(CH2)3-NH2, with terminal amino and ethoxy groups. When
the glass fiber is treated with the silane coupling agent, the ethoxy groups are hydrolyzed to form the
silanol group, –Si–(OH)3, which then condenses with the silanols on the glass fiber to form –Si–O–Si–
covalent bonds. On incorporation of the silane-treated glass fibers into the matrix resin, the unreacted
terminal amino groups react with the epoxy matrix resin. Consequently, the silane coupling agent forms
a molecular bridge between the matrix resin and the fiber reinforcement (Figure 5.11). A number of
OH
GLASS
Si
OH
+
HO
Si
(CH2)3
NH2
OH
O
OH
GLASS
Si
O
Si
(CH2)3
NH2
CH2
CH
Epoxy matrix
OH
OH
GLASS
Si
O
Si
OH
(CH2)3
NH
CH2
C
Epoxy matrix
OH
Figure 5.11 Chemical linkage between matrix resin and glass–fiber reinforcement through a silane coupling
agent.
Copyright 2000 by CRC Press LLC
139
POLYMER MODIFICATION
silane coupling agents are available with reactive groups such as amine, epoxy, or vinyl, which can react
with epoxy, polyester, and other matrix resins during polymer cure. The resulting polymer materials
exhibit improved mechanical properties and durability.
Another illustration of the use of surface modification in the production of improved composite systems
is the surface modification of aramid fibers.12 Poly(p-phenylene terephthalamide) (PPTA) (aramid) fibers
have excellent mechanical properties and good thermal stability. These properties combined with their
low density make aramid fibers prime candidates for reinforcement in high-performance, lightweight
composite materials. However, the usefulness and effectiveness of these fibers in such composite systems
depend on the adhesion between the fiber reinforcement and matrix resin. Unfortunately, the adhesion
between PPTA fibers and most matrices is poor because the chemical inertness and smooth surface
quality of fibers preclude chemical bonding and mechanical anchorage. One of the ways of overcoming
this problem involves chemical modification of the surface of the fibers by introducing functional groups
capable of reacting with the matrix resin.
The surface treatment procedure consists of two stages. In the first step, PPTA fibers are treated with
sulfolane/oxalychloride to produce an intermediate (I) in Equation 5.13.
H
H
O
O
N
N
C
C
+
2nCl
O
O
C
C
Cl
(5.13)
n
N
O
O
N
C
C
+
2HCl
n
C
O
C
O
C
O
C
O
Cl
Cl
I
This highly reactive intermediate can react with a number of compounds to introduce different functionalities on the fiber surface. The resulting functional groups, in the case of reaction with ethylenediamine and glycidol, are shown in Equations 5.14 and 5.15, respectively.
O
N
C
C
C
Cl
O
+
H2N
(CH2)2
NH2
N
C
+ HCl
O
C
O
+
O
C
O
N
H
(CH2)2
NH2
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(5.14)
140
POLYMER SCIENCE AND TECHNOLOGY
O
N
C
C
C
O
+
H2C
O
CH
CH2OH
N
C
O
C
O
O
C
O
+ HCl
(5.15)
O
Cl
CH2
CH
O
CH2
The introduction of these functionalities on the surface of the aramid fiber resulted in substantial
improvement in the fiber–epoxy resin matrix adhesion (about 70%) without a noticeable change in the
tensile strength of the fibers. This suggests first that these were possibly covalent and/or hydrogen bonds
formed between the fibers and the epoxy resin and that the modification was confined to the surface of
the fibers.
Other miscellaneous reactions of polymer have been used to modify the properties of polymers.
Polytetrafluoroethylene and polytrifluoroethylene are extremely inert polymers. However, when films of
these polymers are treated with solutions of Li, Na, Ca, Ba, or Mg in liquid ammonia, they can be
bonded with conventional adhesives to metals and other materials and can be used as backing for pressuresensitive adhesive tapes. The bonds between active hydrogen-containing polymers, such as natural rubber,
and diisocyanate have been enhanced by treatment of the polymer with aliphatic diazo compounds. The
weathering properties of poly(vinyl fluoride) have been increased quite considerably by reacting the
surface of the polymer with a mixture of diisocyanate and a benzoyl compound, thus resulting in a UVabsorbing surface. Poly(ethylene terephthalate) (PET) fabrics have been freed of the objectionable static
charge by reacting PET with 2% NaOH or by reduction with aluminum hydride and subsequently treating
with various diisocyanate–polyglycol reaction products.
IV.
FUNCTIONAL POLYMERS
There is an ever increasing demand for polymers for specific end-use properties such as enhanced
resistance to fire or environmental attack or, in some cases, enhanced degradability. Development of
new polymeric materials to meet this challenge has involved the use of a multidisciplinary approach
involving chemistry, physics, and engineering. Every decade since the middle of 20th century seems to
have witnessed an additional dimension in the thrust of development research in polymer science: the
1950s saw serious advances in polymer chemistry, the 1960s in polymer physics, the 1970s in polymer
engineering, and the 1980s in functional polymers.13 We are currently witnessing the emergence of highperformance polymeric materials such as alloys and blends and advanced composites (so-called polymer
abc) being developed through the application of polymer physics and engineering. In this discussion,
we focus attention on functional polymers.
Functional polymers may be considered in broad terms as those polymers whose efficiency and
characteristics are based on a functional group. A specific functional group is usually carefully designed
and located at a proper place on the polymer chain. The functional groups may be dispersed along the
polymer main chain (including chain ends) or attached to the main chain either directly or via spacer
groups. The main objective of the introduction of special functional groups into the polymer backbone
or side chains is to give the polymers special features. Functional groups are, therefore, typically chemical
units that are chemically reactive, biologically active, electroactive, mesogenic (liquid crystals), photoactive, and, more commonly, ionic, polar, or optically active.13 There are two general techniques used
for the preparation of functional monomers:14
• Polymerization or copolymerization of functional monomers
• Chemical modification of preformed polymers
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POLYMER MODIFICATION
141
The use of functional monomers permits ready control of the content and sometimes the distribution of
units along the polymer chain; this procedure gives more latitude concerning the physical properties of
the final product. On the other hand, chemical modification of an existing polymer, when possible,
enables the choice of molecular weight and the dispersity of the polymer. It also allows the synthesis
of polymers inaccessible by direct route.15
Cross-linking, vulcanization, and grafting are some of the polymer reactions that take place on
functional or reactive groups located in the polymer main chain. We have already discussed some of
these reactions in the preceding sections. We now discuss other examples of modification of polymers
based on reactive monomers, oligomers, or polymers.
A. POLYURETHANES
As we shall see in Chapter 15, polyurethane is a polymer of choice for a wide variety of biomedical
applications. Polyurethane is used extensively in the construction of devices such as vascular prostheses,
membranes, catheters, plastic surgery, heart valves, and artificial organs. Polyurethanes are also used in
drug delivery systems such as the sustained and controlled delivery of pharmaceutical agents, for
example, caffeine and prostaglandin.34
The major reason for the successful application of polyurethanes as biomaterials is their biocompatibility and formulation versatility. The properties of polyurethanes can be controlled over a wide range
by a careful choice of the initial reactants, their ratio, and reaction conditions. Polyurethane elastomers
that are of particular interest in medical technology provide a good illustration of the use of functional
oligomers/polymers with terminal reactive groups. Bifunctional end-terminated oligomers or low-molecular-weight polymers are made by capping polyols — or polyols of polyether, polyester, or polyalkene —
with either aromatic or aliphatic diisocyanate. Typical diisocyanates and polyols used in the synthesis
of these polyurethane intermediates are shown in Tables 5.7 and 5.8, respectively. The isocyanateterminated oligomers (prepolymers or telechelic polymers) are then coupled by reacting with lowmolecular-weight diol or diamine chain extenders. A number of aliphatic and aromatic diols and diamines
used as chain extenders or cross-linking agents in this synthesis are shown in Table 5.8. This two-step
polymerization process (Equations 5.16–5.18), which is usually carried out in solution, results in polyurethane elastomers with good physical characteristics.
Polyurethane elastomers are thermoplastic copolymers of (AB)n type consisting of an alternating
block of relatively long, flexible “soft segment” and another block of highly polar, rather stiff chains or
“hard segment.” The soft segment is derived from a hydroxyl-terminated aliphatic polyester, polyether,
or polyalkene (MW 500 to 5000), while the hard segment is formed from the reaction of diisocyanates
with low-molecular-weight diol or diamine chain extenders. The unique physical and mechanical properties of polyurethane elastomers are determined by their two-phase domain structure. These properties
can be regulated by exploiting the structural variations possible in R, R′, and R″. Therefore, the ultimate
properties of polyurethane-based materials can be controlled largely by the proper choice of the starting
monomers used in their synthesis. For example, caprolactone and/or polycarbonate-based polyesterurethanes are more stable to hydrolysis than those based on polyethylene adipate or oxalate.17,18 Polyetherurethanes exhibit better hydrolytic stability than polyesterurethanes. For hydrophobic urethane elastomers,
propylene oxide is used alone or mixed with a small amount of ethylene oxide. Elastomers with high
resistance to light and thermal and hydrolytic degradation are prepared from hydroxy-terminated homoand copolymers of butadiene and isobutylene, copolymers of butadiene and styrene,19-21 or aliphatic
diisocyanates.22 Polyurethanes produced with aromatic chain extenders are generally stiffer than those
prepared with aliphatic extenders.
Typical medical-grade polyurethanes that have been developed and used successfully in various
extracorporeal and intracorporeal devices include Biomer (Ethicon Inc.); Cardiothane 51 (Kronton Inc.);
and Pellethane (Upjohn/Dow Chemical). For example, resistance to tear and fatigue, as exemplified by
adequate modulus and strength, is essential for polymers to be used in reconstructive surgery of soft
tissue and cardiovascular surgery. Cardiothane is a block copolymer consisting of 90% poly(dimethylsiloxane) (Figure 5.13).16 It has been used as intra-aortic balloons, catheters, artificial hearts, and blood
tubings. This polymer, with its reactive (cross-linkable) acetoxy groups, combines the good mechanical
characteristics of polyurethane with the surface properties of silicone. It further illustrates how the
introduction of reactive groups in the polymer backbone can be used to produce a polymer with specific
end-use properties.
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POLYMER SCIENCE AND TECHNOLOGY
The chemical and/or biological modification of polyurethane surfaces, such as grafting of hydrogels
like acrylamide or poly(hydroxyethyl methacrylate) enhances blood compatibility. Biocompatibility and
blood compatibility can be improved by treating the polymer surface with a solution of albumin or
gelatin followed by cross-linking with gluteraldehyde or formaldehyde.23
Example 5.6: The tendency for hard segment–soft segment phase separation in urethane copolymers
increases in the order: polybutadiene and/or polyisoprene soft segment >> polyether soft segment >
polyester soft segment. Explain.
Solution: Polyurethane elastomers are segmented copolymers composed of alternating soft and hard
blocks. Hydrogen bonding in polyurethanes occurs between the NH hydrogens in the hard segment and
the carbonyl oxygens in the hard segment or between the carbonyl and ether oxygens in the soft segment
(Table E5.6).
Table E5.6 Possible Hydrogen Bonding in Polyurethane
O
O
C
O
H
N
C
O
N
H
O
C
N
H
R
O
O
O
Urethane-Urethane
Urethane-Ester
O
N
H
C
O
R
C
O
H
N
N
H
O
N
H
O
C
H
N
R
Urethane-Ether
Urea-Urea
O
N
H
C
O
H
N
C
O
R
N
H
O
C
N
H
O
H
N
Urethane-Urea
Copyright 2000 by CRC Press LLC
C
R
Urea-Ether
143
POLYMER MODIFICATION
Table E5.6 (continued) Possible Hydrogen Bonding
in Polyurethane
N
H
C
O
N
H
R
O
C
O
Urea-Ester
The degree of microphase separation, therefore, depends on the relative magnitudes of intra- and
interphase H-bonding. The strength and degree of hard segment–soft segment H-bonding are greater for
the carbonyl group than the ether oxygen due to its relative polarities. Consequently, hard segment–soft
segment compatibility is higher with polyester soft segment than polyether soft segment. With polybutadiene and/or polyisoprene soft segments, the potential for hard segment–soft segment H-bonding is
nonexistent; only van der Waals forces operate between the two phases. Consequently, the tendency for
phase separation is pronounced.
B. POLYMER-BOUND STABILIZERS13,24
Traditionally, stabilizers for polymers such as antioxidants, ultraviolet stabilizers, and flame retardants
are low-molecular-weight compounds. However, several problems are associated with the use of these
stabilizers. These include their loss from the finished product by evaporation or leaching during fabrication. Also, compatibility at higher levels of incorporation of the stabilizers may be problematic. Another
problem is the potential danger of toxicity. Low-molecular-weight chlorine, bromine, or phosphorous
compounds commonly used as flame retardants, as well as phenolic derivatives that are frequently
employed as antioxidants or ultraviolet stabilizers, are potentially toxic. Oligomeric or, possibly, polymerbound stabilizers have the potential for eliminating these problems. These are prepared by synthesizing
suitable functional monomers followed by homo- and/or copolymerization.
1. Antioxidants
2,6-ditertiarybutyl-1,4-vinyl phenol or 4-isopropenyl phenol polymerizes readily with isoprene, butadiene, styrene, and methyl methacrylate (Equation 5.19).
The resulting copolymers are good antioxidants for their parent polymers at copolymer compositions of
only 10 to 15 mol% of polymerizable antioxidant.
2. Flame Retardants
Flame retardants are usually halogen-containing materials. 2,4,6-Tribromophenyl, pentabromophenyl,
and 2,3-dibromopropyl derivatives of acrylate and methacrylate esters can be readily polymerized or
copolymerized with styrene, methyl methacrylate, and acrylonitrile to produce polymers with improved
flame retardancy (Equation 5.20).
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POLYMER SCIENCE AND TECHNOLOGY
Table 5.7 Typical Polyols Used in Syntheses of Polyurethane Elastomers
Polyols
HO
(CH2CH2
O
HO
(CH2CH2CH2CH2
Polyethylene oxide (PEO)
)n H
O
Polytetramethylene oxide (PTMO)
)n H
CH3
HO
(CH2CH
O
)n H
(CH3)2
CH2
HO
C
HO
(CH2)4
O
HO
(CH2)3
CO
HO
(CH2)4
HO
CH2
CH
Polypropylene oxide (PPO)
Polyisobutylene (PIB)
OH
n
OC(CH2)CO
O
n
Si(CH3)2
O
CH
CH2
O
n
Polyethylene adipate (PEA)
H
Polycapropactone (PCL)
H
n
CH
Si(CH3)4
OH
CH2CH
CH
CH
Polydimethylsiloxane, hydroxybutyl
terminate (PDMS)–OH
CH2
OH
Polybutadiene (PBD)
CH2
From Gogolewski, S., Colloid Polym. Sci., 267, 757, 1989. With permission.
Table 5.8 Typical Chain Extenders Used in Syntheses
of Polyurethane Elastomers
Chain Extenders
HO
(CH2)4
OH
1,4-Butanediol (BD)
HO
(CH2)6
OH
Hexanediol (HD)
HO
(CH2)2
OH
Ethylene diol (ED)
HO
(CH2)2
O
H2N
(CH2)2
NH2
(CH2)2
OH
Diethylene diol (DED)
Ethylenediamine (EDA)
CH3
H2N
CH2
CH
NH2
Propylenediamine (PDA)
From Gogolewski, S., Colloid Polym. Sci., 267, 757, 1989. With
permission.
Copyright 2000 by CRC Press LLC
2O
C
N
R
N
C
O + HO
(R´)
OH
O
C
N
R
H
O
N
C
O
(R´)n
O
O
H
C
N
R
N
C
(5.16)
Prepolymer (macrodiisocyanate)
O
C
N
R
H
O
N
C
O
(R´)n
O
O
H
C
N
R
N
C
O
+
HO
R´´
Prepolymer
O
H
C
N
H
O
N
C
O
OH
diol
O
H
C
N
H
O
N
C
(5.17)
O
R´´
O
R
O
(R´)n
O
R
O
R´´
O
x
Polyurethane
O
C
N
R
H
O
N
C
O
(R´)n
O
O
H
C
N
R
N
C
O + H2 N
Prepolymer
H
N
R´´
NH2
diamine
H
O
H
N
C
N
R
H
O
N
C
O
(R´)n
O
O
H
C
N
Polyurethane urea
Figure 5.12 Equations representing the two-step process for producing polyurethane copolymers.
Copyright 2000 by CRC Press LLC
R´´
R
H
O
H
N
C
N
H
R´´
(5.18)
N
x
146
POLYMER SCIENCE AND TECHNOLOGY
(CH2)4
O
O
H
C
N
φ
H
O
N
C
φ
CH2
O
(CH2)4
O
O
H
C
N
φ
n
φ
CH2
H
O
N
C
O
Base polyurethane chain
X
X
(X) - Hydrogens in hydrocarbon portions of molecule
O
O
H
+ CH3C
O
CH3
O
Si
Si
CH3
O
C
CH3
CH3
C
CH3
O
Base polymer
Acetoxy-terminated
silicone crosslinking agent
H2O
CH3
O
Si
Si
CH3
OH
CH3
+
Crosslinked polymer
HO
C
CH3
Acetic acid
Figure 5.13 Structure of Cardiothane. (From Gogolewski, S., Colloid Polym. Sci., 267, 757, 1989. With permission.)
R
CH2
R
C
+
CH2
X
AIBN
R´
R
H, CH3, R´
X
OH
Copyright 2000 by CRC Press LLC
C
R
CH2
C
X
COOCH3,
R
CH2
n
(5.19)
C
R´
m
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POLYMER MODIFICATION
R
CH2
C
R
+
CH2
COOX
R
R
C
CH2
M
C
CH2
COOX
M
n
CH2CH2Br
Br
Br
m
Br
CH2Br,
Br
Br
Br
M
(5.20)
C
H, CH3
Br
X
R
COOCH3,
N
N
CN;
O
N
Br
Polymerizable groups
CR
OCO
CH2
CR
CH2
OCH2CHOHCH2OCOCR
CH2NHCOCH
R
CH2
CH2
H, CH3
Figure 5.14 Polymerizable 2(2-hydroxyphenyl) 2H-benzotriazole derivatives.
Incorporation of only about 5% of these copolymers in polystyrene increased the limited oxygen index
by 30%.13 This is substantially lower than the 10 to 12% of the usual flame retardant materials required
to protect polystyrene and similar polymers from being highly inflammable.
3. Ultraviolet Stabilizers
Upon exposure of polymeric materials to sunlight, ultraviolet stabilizers tend to evaporate from their
surfaces where, incidentally, the stabilizing action is particularly required. 2(2-Hydroxyphenyl) 2Hbenzotriazole derivatives (Figure 5.14) have been recognized to be the most effective compounds for
protecting polymeric materials from ultraviolet and photodegradation. Small quantities (a few percentage
points) of photostabilizers have been introduced into polyesters, polycarbonates, and polyamide using
hydroxy-, acetoxy-, carboxyl-, and carbomethoxy-derivatives of this compound. These polymer-bound
stabilizers are substantially more effective than the lower molecular weight UV stabilizers. The modified
polyester has been synthesized directly. For example, poly(ethylene sebacate) was made from dimethyl
sebacate and ethylene glycol with 1 to 3% of ethylene glycol replaced by 2(2,4-dihydroxyphenyl) 2Hbenzotriazole (Equation 5.21). Similarly, ultraviolet-stabilized polycarbonate was made using bisphenol
A and diphenyl carbonate (Equation 5.22).
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POLYMER SCIENCE AND TECHNOLOGY
OH
HO
Bzt
O
O
(5.21)
OH
+
CH3O
OH
HO
Bzt
Bzt
C
R
C
OCH3
+
HO
R´
OH
OH
OH
O
O
OH
R´
O
C
R
C
O
Bzt
O
O
or
O
O
C
R
O
C
O
O
O
OH
R´
O
C
R
C
O
Bzt
Bzt
O
C
R
C
O
OH
N
Bzt
N
N
R´
Copyright 2000 by CRC Press LLC
(CH2)n , n = 4, 6, 10
;
R
(CH2)8 ,
,
;
149
POLYMER MODIFICATION
OH
HO
Bzt
O
OH
+
Ph
O
C
O
Ph
+
HO
R´
(5.22)
OH
OH
HO
Bzt
Bzt
OH
OH
O
OH
R´
O
C
O
Bzt
O
or
O
C
O
O
O
OH
R´
O
C
O
Bzt
Bzt
O
C
O
OH
CH3
N
Bzt
N
N
;
R´
C
CH3
The discussion so far involves cases of functional polymers where the reactive/functional groups are
attached directly to the backbone. The reactivity of a functional group directly attached to the polymer
backbone depends on the proximity, flexibility, and polarity of the polymer main chain. The interference
from the polymer main chain can be eliminated, or at least reduced, through functional groups attached
as side groups or through the use of spacer groups inserted between the reactive groups and the polymer
main chain. For example, in addition to the techniques discussed above, the ultraviolet stabilizer (2-(2hydroxyphenyl) 2H-benzotrizole can be incorporated in the polymer structure using functional monomers
or polymers with reactive side groups: the ring-opening reaction of the oxirane ring of glycidyl methacrylate followed by copolymerization with methyl methacrylate produces UV-stabilized PMMA
(Equation 5.23).13
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150
POLYMER SCIENCE AND TECHNOLOGY
R
N
O
N
+
CH2
N
CH
CH2
OH
R
O
C
CH2
O
CH3
TBA Br
C6H6
OH
N
N
AIBN
N
O
CH2
CH
CH2
O
O
OH
CH2
( CH2
C
C
CH2
R´
CH2
CH3
C
( CH2
C
O
(5.23)
R´´
R´
)
C
C
)
R´´
O
CH2
CH
R = H, OMe, Cl
R´ = H, CH3
R´´ = COOMe, C6 H5
CH2
O
N
OH
N
N
OH
R
PMMA containing a small amount of glycidyl methacrylate units, when allowed to react with tetraalkyl
ammonium salts, will undergo ring-opening reaction with 2(2,4-dihydroxyphenyl) 2H-benzotriazole
(Equation 5.21).
N
R
( CH2
C
CH3
)
( CH2
C
C
O
)
C
N
O
CH2
R = H, CH3
CH
R
( CH2
C
CH
TBA Br
C6H6
(5.24)
CH3
)
C
OMe
OH
O
O
OMe
N
+
( CH2
C
O
)
C
+
O
O
CH2
CH
CH2
O
N
OH
N
OH
N
Natural polymers very frequently link functional groups via spacer groups. For example, the amino
acids lysine and glutamic acid have some of their functional groups linked by spacer groups (Figure 5.15).
In synthetic polymer, spacer groups might be stiff groups like the phenyl group in p-substituted polystyrenes used in ion-exchange resins or, as is usually the case, flexible groups such as methylene or
fluorocarbon groups. Fluorocarbon ether groups have been used successfully to separate sulfonate or
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151
POLYMER MODIFICATION
NH2
H2N
CH2
CH2
CH2
CH2
C
COOH
H
Lysine
NH2
HOOC
CH2
CH2
C
COOH
H
Glutamic acid
Figure 5.15 Spacer groups in amino acids.
carboxylate groups from the polytetrafluoroethylene main chain to produce fluorocarbon polymer membrane of unusual properties.24 Polymers have also been synthesized with spacer groups that have
polyethylene, polyoxyethylene, and even the much stiffer polymethacrylate backbone. Polymethacrylates
have been particularly useful as polymers with functional groups that can be separated effectively from
the polymer main chain. Polymers and copolymers based on hydroxyethyl methacrylate or glycidyl
methacrylate, for example, have been used in biomedical applications.
C. POLYMERS IN DRUG ADMINISTRATION28,30
The medical field is no longer imaginable without synthetic polymers. They are used both as implant
and surgical materials and as drugs or carriers for drugs. Drugs are hardly ever administered to patients
in an unformulated state. Polymers have long been employed as excipient for adjusting the consistency
and release properties of simple solid, cream, and liquid formulations. Polymers that have typically been
used in drug delivery include various cellulose derivatives, polyacrylates, poly(vinyl pyrrolidone), polyoxyethylene, poly(vinyl alcohol), and poly(vinyl acetate). Poly(vinyl pyrrolidone), poly(2-hydroxypropyl
methacrylamide), and polyoxazoline are examples of synthetic polymers that have been used as plasma
expanders.28 We discuss a few examples to illustrate the use of functional polymers in drug administration.
The traditional approach in pharmaceutical research is to focus effort on the discovery of new
compounds with biological activity that can be used in the treatment of diseases. The main problem
associated with this approach is that the drug may be distributed to a variety of sites in the body where
it may be inactive, harmful, and/or toxic. Both the therapeutic and harmful/toxic effects of drugs depend
on their concentrations at the various sites in the body.28 There is normally an optimal therapeutic range
within which the therapeutic effects of the drug outweigh its harmful effects. Outside this range, the
drug may either be inactive or its harmful effects may predominate (Figure 5.16).
Two distinct approaches are currently used to improve drug action through its mode of delivery. These
are controlled drug release and site-directed or targeted drug delivery.
Figure 5.16 A schematic plot of different drug administration routes. (A) Single dose preparation; (B) sustained
release preparation; (C) prolonged release preparation.
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POLYMER SCIENCE AND TECHNOLOGY
1. Controlled Drug Release
Most drugs are low-molecular-weight compounds, which when administered conventionally are often
rapidly excreted from the body. Intermittent doses result in a sawtooth pattern of the drug in circulation
due to the rise and fall of the concentration of the drug. Consequently, large and repeated doses are
required to sustain a therapeutic effect. The aim of controlled drug release, therefore, is to eliminate or
at least reduce the danger of overdose, with risk of side effects or subtherapeutic blood levels, by
producing and maintaining an optimal therapeutic concentration of the drug in the body. Most controlledrelease delivery systems rely on a polymer to regulate the flow of the therapeutic agent out of the device.
A number of techniques are used to achieve this objective, including encapsulation, dispersion in
hydrophobic vehicles or porous polymers, binding of the drug to macromolecules, and formation of
drug–carrier complexes. We illustrate a couple of these techniques.
a. Tablets with Prolonged and Sustained Drug Release29
The overall release profile of a drug can be controlled by the physical shape of the device or by the
chemical nature of the polymer matrix. Tablets with a sustained and prolonged delivery have been
prepared using polymers and terpolymers based on acrylic and methacrylic acids as coatings for some
dosage forms. The behavior of these polymeric materials in the various parts of the gastrointestinal tract
varies depending on the chemical structure and molar mass and on the ratio of basic, acidic, and
hydrophobic groups. Thus the location and rate of release of the active substance can be controlled.
Figure 5.17 shows a typical cationic copolymer. It is basic, swells in the pH range of 5 to 8 but is
soluble at pH 2.5. The anionic polymers Eudragit L, S (Figure 5.18) are based on methacrylic acid and
methacrylic acid methyl ester. They are resistant to gastric juice, but soluble in the small intestine and
insoluble and impermeable to water below pH 6 to 7 range. They consequently provide full protection
against gastric acid and ensure drug release in the small intestine.
Another variation of these acrylic copolymers are acrylic terpolymers based on methacrylic acid with
a small amount of quaternary ammonium group. These terpolymers are water insoluble in the entire
physiologic field. They, however, swell and are permeable to water and drugs. Their permeability is
independent of pH and can be controlled by appropriate structural modification. The polymeric films
behave essentially as semipermeable membranes with a domain microstructure: the predominant hydrophobic parts of the macromolecule cause insolubility in water, while the hydrophobic quaternary groups
permit the passage of water and drug molecules.
R
CH2
R
C
CH2
C
C
O
C
(CH2)
N
O
CH3
R´
O
R
H, CH3
R´
CH3, C4 H9
CH3
Figure 5.17 Eudragit E-cationic polymer. (From Kalal, J., Makromol. Chem. Macromol. Symp., 12, 259, 1987.)
CH3
CH2
Figure 5.18 Eudragit anionic polymers: (L) –COOH/–OCH3 = 1/1;
(S) –COOH/–OCH3 = 1/2. (From Kalal, J., Makromol. Chem. Macromol.
Symp., 12, 259, 1987.)
Copyright 2000 by CRC Press LLC
C
CH2
C
C
O
C
OH
O
CH3
O
153
POLYMER MODIFICATION
b. Degradable Polymers13
Polymers with functionalities in the main chain very often have aromatic groups that consequently cause
the polymers to become infusible, intractable, and nonfabricable.30,31 For example, the copolycarbonates
or bithionol and glycols where the glycol units are small are essentially intractable. However, by using an
appropriate spacer group, the hydrophilicity of the polymer can be modulated. This is achieved by using
bithionol — an antibacterial agent — and a bisphenol as a comonomer in an alternating copolycarbonate
with a poly(ethylene oxide) glycol with a DP of at least 10 (Equation 5.24). The resulting polycarbonate
has useful physical characteristics. In addition, it is degradable under physiologic conditions for controlled
release of the bactericide bithionol and of the carbon dioxide and poly(ethylene oxide) glycol.
O
Cl
C
O
O
O
C
Cl
S
+
Cl
HO ( CH2CH2
O )m H
Cl
(5.25)
O
C
O
O
C
O ( CH2CH2
O )m
n
S
Cl
Cl
Polycarbonate of bithionol with poly(ethyleneoxide) glycol as spacer group
2. Site-Directed (Targeted) Drug Delivery
The basic principle in site-directed or targeted drug delivery is that the carrier will recognize the diseaserelated target or biochemical site of action, interact with it, and concentrate delivery of the drug at the
site. The drug is thus rendered inactive elsewhere in the body. The polymer in targeted drug delivery
systems acts merely as a carrier with no intrinsic pharmacologic activity or therapeutic effect. In other
words, the macromolecule is chemically modified with molecules of the drug and other moieties to
overcome some undesirable characteristics of the drug, but can be transformed by a chemical or enzymatic
process to release the pharmacologically active drug at the appropriate time or location (Figure 5.19).
To satisfy this requirement the macromolecule must be designed to accommodate features that may even
be in conflict. Some of these features are28
• It must provide overall solubility to the targeting system, and after the drug release the carrier itself
must be both water soluble and capable of being totally excreted by the body after the required period
of time.
• It may need to contain groups available for the chemical attachment of drugs via biodegradable linkages.
• It must contain structural features necessary for interaction specifically with the discrete features of
the biosystem.
• It is expected to prevent nonspecific interactions between the components of the biological system and
polymer targeting system.
In general, the properties of a biosystem and a synthetic polymer as well as the nature of the biological
medium dictate the degree and type of interaction between a biostructure and a polymer. The biocompatibility of synthetic polymers depends on their chemical nature, physical state, and macroscopic form,
which can be modified by functionalization of the polymer skeleton. Many biopolymers, such as proteins
and nucleic acids, are natural polyelectrolytes. Similarly, the outer cell membrane of living cells has
charged groups. The biological medium is an electrolyte with an aqueous phase. Therefore, electrostatic
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154
POLYMER SCIENCE AND TECHNOLOGY
Figure 5.19 Generalized structure of soluble polymeric drug carriers: (D) drug; (S) spacer group; (L) and (W)
groups that determine the solubility of macromolecule in nonaqueous and aqueous environment, respectively;
(T) a group specifically recognized by an organ/tissue/cell of the body. (From Petrak, K., Br. Polym. J., 22, 213,
1990. With permission.)
forces play a dominant role in the reversible interaction of biopolymer with synthetic polymers, while
covalent bonding serves the irreversible immobilization of biostructures.32
Most polymer carriers per se do not possess the ability to recognize the target site. Consequently,
biological macromolecules such as polysaccharides, antibodies, and toxin fragments are introduced into
the polymer carrier as the directional component and to ensure the required selectivity. The functional
groups of the polymer are usually modified or activated to effect coupling of the biomolecule (Table 5.9).
Sometimes spacer groups are used to increase the distance between the polymer carrier and the
biostructure so as to reduce interference from the polymer main chain and maintain proper functional
properties. While there are currently no efficient site-directed drug delivery systems available for practical
clinical applications, the feasibility of the general concept has been amply demonstrated. Examples are
prodrugs based on N-(2-hydroxypropyl)-methacrylamide copolymers containing oligopeptide side chains
terminating in an anticancer drug, Adriamycin (Figure 5.20). The bond between the drug and the carrier
is stable in the bloodstream, but is cleared intracellularly on exposure to appropriate enzymes.29
Table 5.9 Chemical Modification of Synthetic Polymers for Covalent Linkage
of Biomacromolecules and Affine Ligands
Functional Group
of the Polymer
–OH
–COOH, –NH2 Carbodiimides
–CHO
–C6H4–R
–SH
Chemical Modification
(activation) of the Polymer
Cyanogen bromide
Triazines
Periodate oxidation
Benzoquinones
Epoxides
Silanization
Carboxylating reagents
–NH2, –COOH
Acylation
Active esters
Diazotation reagents
Thiol-disulfide exchange
Binding Group of the
Biomacromolcule and Ligand
–NH2
–NH2
–NH2
–NH2
–NH2, –COOH, –OH,–SH
–NH2, –COOH, –C6H5-R
–NH2
–NH2, –COOH
–NH2, –COOH
–NH2
–C6H4–R, histidine, tryptophan
–SH
From Kalal, J., Makromol. Chem. Macromol. Symp., 12, 259, 1987.
V. PROBLEMS
5.1. When two homopolymers with the same melt index are blended together by the addition of a compatibilizing agent, the resulting blend has a higher melt index. Explain.
Copyright 2000 by CRC Press LLC
155
POLYMER MODIFICATION
Figure 5.20 Schematic diagram of N-(2-hydroxypropyl)-methacrylamide copolymers containing oligopeptide
side chains terminating in anticancer drugs or targeting moieties. (From Kalal, J., Makromol. Chem. Macromol.
Symp., 12, 259, 1987.)
5.2. Dextrins are degradative products of starch obtained by heating starch in the presence or absence of hydrolytic
agents. Typical conditions for the white and yellow (canary) dextrins are shown in the following table.
Manufacturing
Condition
White Dextrins
Canary Dextrins
Usual catalyst
Temperature, °C
Time, h
HCl
79–121
3–7
HCl
149–190
6–20
Envelope front seal adhesives are made from solutions of dextrins in water. They require high solid contents,
usually 60 to 70%. Which of the two dextrin types (white or yellow) would be more suitable as adhesives for
envelope seals?
5.3. Indicate how you would increase:
a. The moisture resistance of nylon 6
b. The toughness and crack resistance of polyethylene (HDPE)
c. Impact resistance of polystyrene
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156
POLYMER SCIENCE AND TECHNOLOGY
5.4. Explain the observed trends in the data for the melt index and the peel strength to aluminum of
polyethylene (PE) and ethylene–acrylic acid copolymer (EAA) shown in the following table.
Polymer
Acrylic acid
(mol%)
Melt Index
(g)
Peel Strength
(lb/in.)
PE
EAA
EAA
EAA
0
4.6
6.0
8.2
1.5
83
58
52
<1
56
81
100
5.5. Nitrile rubbers possess outstanding oil-resistant properties. Explain the following variation in properties
of nitrile rubber.
Acrylonitrile Content
in Copolymer (%)
Properties of Nitrile Rubber
2–5
15
25
35–40
50–60
Rubbery with poor oil resistance
Rubbery with fair oil resistance
Rubbery with medium oil resistance
Leathery wit high oil resistance
Tough, leathery plastic with high resistance to aromatics
5.6. Suggest a possible method to effect the following changes through copolymerization. Briefly explain
the basis of your suggestion.
i. Increase:
(a) the rigidity of O
(CH2)20
n
(b) the chemical resistance of CH2
(c) the flexibility of
(Str. 11)
CO
(Str. 12)
CH
n
O
H
C
N
(Str. 13)
n
(d) adhesion to polar substrates of CH2
CH2
H
(e) the temperature resistance of N
(CH2)6
(Str. 14)
n
H
O
N
C
O
(CH2)
(Str. 15)
C
n
ii. Obtain cross-linked polymers from:
(a)
Copyright 2000 by CRC Press LLC
CH2
(Str. 16)
CH
n
157
POLYMER MODIFICATION
(b)
O
O
C
C
O
CH2
CH2
(Str. 17)
O
5.7. An unsaturated polyester based on maleic anhydride is to be used in an application requiring good creep
resistance. Which of the available unsaturated monomers (methyl methacrylate or vinyl acetate) would
you choose for curing the polyester? Explain your choice.
5.8. The nature of cross-linked urea–formaldehyde (UF) resins is thought to be
CH2
NH
CO
N
CH2
NH
CO
NH
CH2
NH
CO
NH
CH2
(Str. 18)
N
CO
NH
CH2
Films from cured UF resins are very brittle. Arrange the following components in their increasing ability
to reduce the brittleness of UF films through copolymerization with urea and formaldehyde.
(a)
H2N
CO
NH
(b)
H2N
CO
NH
(c)
H2N
CO
NH
(d)
H2N
CO
NH
(CH2)12
NH
CO
NH
NH
(CH2)6
NH
CO
CO
(Str. 19)
NH2
CO
(Str. 20)
NH2
(Str. 21)
NH2
(Str. 22)
NH2
5.9. Polyimides are formed by the reaction:
O
O
O
O
C
C
C
C
O
O +
H2N
NH2
N
N
C
C
C
C
O
O
O
O
n
(Str. 23)
Polyimides are insoluble, infusible polymers that cannot be used as adhesives. Which of the following
two compounds would you copolymerize with the starting monomers to produce a useful adhesive?
Comment on the temperature resistance of the resulting adhesive.
H2N
NH2
H2N
H2N
NH2
H2N
(A)
Copyright 2000 by CRC Press LLC
O
NH2
NH2
(B)
(Str. 24)
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POLYMER SCIENCE AND TECHNOLOGY
5.10. How much sulfur, in parts per hundred resin (phr), would be required to vulcanize polybutadiene rubber
of molecular weight 5400 assuming:
a. Disulfide cross-links
b. Decasulfide cross-links
5.11. UF resins are susceptible to hydrolytic degradation. Rank the following monomers in order of the
expected increase in the hydrolytic stability of UF resins modified by the incorporation of these
monomers through copolymerization of their urea derivatives. Explain the basis of your ranking order.
Monomer
Structure
1. Hexamethylenediamine
2. Trimethylene oxidediamine
3. Dodecanediamine
H2N–(CH2)6–NH2
H2N–CH2CH2O–CH2CH2OCH2CH2-NH2
H2N–(CH2)12–NH2
5.12. The stiffness of the polymer backbone chain has been found to be responsible for the minimum length
of spacer groups required to make the reactivity of the functional group free from the interference of
the polymer main chain. For poly(ethylene oxide) three to four methylene groups are required, whereas
spacer groups of one or two methylene are necessary for polyethylene. Explain this observation.
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I., Ed., Van Nostrand Reinhold, New York, 1977.
2. Wake, W.C., Adhesion and the Formulation of Adhesives, Applied Science Publishers, London, 1976.
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New York, 1977.
6. Fettes, E.M., Ed., Chemical Reactions of Polymers, Interscience, New York, 1964.
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New York, 1976.
8. Ceresa, R.J., Block and graft copolymers, in Encyclopedia of Polymer Science and Technology, Mark, H., Gaylord,
N.G., and Bikales, N.M., Eds., Interscience, New York, 1976.
9. Gaylord, N.G., and Aug, F.S., Graft copolymerization, in Chemical Reactions of Polymers, Fettes, E.M., Ed.,
Interscience, New York, 1964.
10. Angier, D.J., Surface reactions, in Chemical Reactions of Polymers, Fettes, E.M., Ed., Interscience, New York, 1964.
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Blomguist, R.F., Christiansen, A.W., Gillespie, R.H., and Myers, G.E., Eds., Pennsylvania State University, 1983.
12. Merx, F.P.M. and Lemstra, P.J., Polym. Commun., 31(7), 252, 1990.
13. Vogl, O., Jaycox, G.D., and Hatada, K., J. Macromol. Sci. Chem., A27(13 and 14), 1781, 1990.
14. Prechet, J.M.J., Deratani, A., Darling, G., Lecavalier, P., and Li, N.H., New reactive polymers containing nitrogen
functionalities from asymmetric synthesis to supported catalysis, Makromol. Chem. Macromol. Symp. 1, 91, 1986.
15. Soutif, J. and Brosse, J., Chemical modification of polymerizations. I. Applications and synthetic strategies, React.
Polym. 12, 3, 1990.
16. Gogolewski, S., Colloid Polym. Sci., 267, 757, 1989.
17. Frisch, K.C., Advances in Urethanes, seminar materials, Technomic, Westport, CT, 1985.
18. Speckhard, T.A., Verstrate, G., Gibson, P.E., and Cooper, S.L., Polym. Eng. Sci., 23, 337, 1983.
19. Rausch, K.W., McClellan, T.R., Jr., d’Ancicco, V.V., and Sayigh, A.A.R., Rubber Age, 99, 78, 1967.
20. Verdol, J.A., Carrow, D.J., Ryan, P.W., and Kuncl, K.L., Rubber Age, 98, 62, 570, 1966.
21. Chang, V.S.C. and Kennedy, J.P., Polym. Bull., 8, 69, 1982.
22. Mennicken, G.J., Oil Colour Chem. Assoc., 49, 639, 1966.
23. Imai, Y. and Nose, K., Trans. Am. Soc. Artif. Intern. Organs, 17, 6, 1971.
24. Vogl, O. and Sustic, A., Makromol. Chem. Macromol. Symp., 12, 351, 1987.
25. Thijs, L., Gupta, S.G., and Neckers, D.C., J. Org. Chem., 44, 4123, 1979.
26. Gupta, S.N., Thijs, L., and Neckers, D.C., J. Polym. Sci. Polym. Chem. Ed., 19, 855, 1981.
27. Gupta, S.N., Thijs, L., and Neckers, D.C., Macromolecules, 13, 1037, 1980
28. Petrak, K., Br. Polym. J., 22, 213, 1990.
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POLYMER MODIFICATION
29.
30.
31.
32.
33.
34.
159
Kalal, J., Makromol. Chem. Macromol. Symp., 12, 259, 1987.
Cho, B.W., Jin, J.I., and Lenz, R.W., Makromol. Chem. Rapid Commun., 3, 23, 1982.
Jin, J.I., Antoum, S., Ober, C., and Lenz, R.W., Br. Polym. J., 12, 132, 1982.
Scheler, W., Makromol. Chem. Macromol. Symp., 12, 1, 1987.
Greek, B.F., Chem. Eng. News, p. 25, March 21, 1988.
Batyrbekov, E.O., Moshkeich, S.A., Rukhina, L.B., Bogin, R.A., and Zhubanov, B.A., Br. Polym. J., 23(3), 273, 1990.
Copyright 2000 by CRC Press LLC
Chapter 6
Condensation (Step-Reaction) Polymerization
I.
INTRODUCTION
Condensation polymerization is chemically the same as a condensation reaction that produces a small
organic molecule. However, as we saw in Chapter 2, in condensation polymerization (i.e., production
of a macromolecule) the functionality of reactants must be at least 2. Recall that functionality was defined
as the average number of reacting groups per reacting molecule. To derive expressions that describe the
physical phenomena occurring during condensation polymerization (polycondensation) — a tool vital
to process design and product control — three approaches have been traditionally adopted: kinetic,
stoichiometric, and statistical. Various degrees of success have been achieved by each approach. We treat
each approach in the succeeding sections. Before then, we briefly discuss the overall mechanism of
polycondensation reactions.
II.
MECHANISM OF CONDENSATION POLYMERIZATION
The mechanism of polycondensation reactions is thought to parallel that of the low-molecular-weight
analogs. As a result of their macromolecular nature, polymers would be expected to have retarded
mobility. It was therefore predicted, purely on theoretical arguments, that the chemical reactivity of
polymers should be low.
• The collision rate of polymer molecules should be small due to their low kinetic velocity. This should
be accentuated by the high viscosity of the liquid medium consisting of polymer molecules.
• Shielding of the reactive group within the coiling chain of its molecule should impose steric restrictions
on the functional group. This would lead to a reduction in the reactivity of the reactants.
Flory1 has shown from empirical data that for a homologous series the velocity constant measured
under comparable conditions approaches an asymptotic limit as the chain length increases. He therefore
proposed the equal reactivity principle, which states in essence that “the intrinsic reactivity of all
functional groups is constant, independent of the molecular size.” This principle is in apparent contradiction to the theoretical prediction of low chemical reactivity for macromolecules discussed above.
Flory used the following arguments to explain the equality of reactivity between macromolecules and
their low-molecular-weight analogs.
1. Low diffusivity of macromolecules — Undoubtedly, large molecules diffuse slowly. But the gross
mobility of functional groups must not be confused with the overall diffusion rate of the molecule as
a whole. The terminal group, though attached to a sluggish moving polymer molecule, can diffuse
through a considerable region due to the constant conformational changes of polymer segments in its
vicinity. The mobility of the functional group is much greater than would be indicated by the macroscopic viscosity. Thus it was concluded that the actual collision frequency had little relation to the
molecular mobility or macroscopic viscosity.
A pair of neighboring functional groups may collide repeatedly before they either diffuse apart or
react. The lower the diffusion rate, the longer these groups remain in the same vicinity. Consequently,
the number of collisions will be greater and hence the probability of fruitful collisions will be greater.
However, by the same token, once reactants diffuse apart, it will take a proportionately longer time
before two reactants meet to engage in another series of collisions. Flory therefore concluded that for
a sufficiently long time span, the decreased mobility resulting from larger molecular size and/or high
viscosity will alter the time distribution of collisions experienced by a given functional group but not
the average number of collisions.
2. Steric factor — Flory expected steric-factor-related reduction in chemical reactivity due to shielding
of functional groups by coiling of their chain to occur only in dilute solution where sufficient space is
0-8493-????-?/97/$0.00+$.50
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161
162
POLYMER SCIENCE AND TECHNOLOGY
available for molecules to coil independently. In concentrated systems, however, polymer chains
intertwine extensively. Functional groups show no preference for their own chains. The chains act as
diluents whose effects can be accounted for by writing rate expressions in terms of concentration of
functional groups and not in terms of molecules or mole fractions of molecules.
Example 6.1: The esterification of a monobasic acid by alcohol proceeds according to the following
reaction.
H(CH2)10COOH + C2H5OH
HCl
H(CH2)10COOC2H5 + H2O
(Str. 1)
The rate of ester formation is given by the equation:
d [ester group]
= k [COOH] H +
dt
[ ]
For carboxyl group and HCl concentrations of 103 and 10–2 g-eq/l, respectively, the rate of esterification
was found to be 7.6 × 10–3 g-eq/l/s. For the esterification of the dibasic acid (CH2)1000(COOH)2, how
much ester is formed in a 8-h day shift using a 100-l reactor if the carboxyl group concentration is
102 g-eq/l and the acid (HCl) concentration is 10–3 g-eq/l?
Solution: Notes:
1. The reactivity of one carboxyl group in the dibasic acid is unaffected by esterification of the other.
Consequently, the same rate equation holds for the esterification of both monobasic and dibasic acids.
2. The rate equation is applicable to the esterifications of all polymethylene dibasic acids for which n is
unity or greater.
From the above two observations, it follows that esterification rate constant for homologous series of
dibasic acid is the same as those for monobasic acids.
g ⋅ eq −2 g ⋅ eq
g ⋅ eq
Rate = k 10 3
= 7.6 × 10 −3
10
l ⋅s
liter
liter
g ⋅ eq
k = 7.6 × 10 −4
l
−1
1
⋅
s
l 1 2 g ⋅ eq −3 g ⋅ eq
10
Rate (dibasic acid) = 7.6 × 10 −4
10
g ⋅ eq s
l
l
7.6 × 10 −5 g ⋅ eq 100 l 8 × 3600 s
(
)(
)
l⋅ s
= 7.6 × 10 −5 × 100 × 28800 g ⋅ eq
= 7.6 × 28.8 g ⋅ eq
Concentrations are written as equivalents of functional groups. (CH2)1000(COOC2H5)2: gram equivalent
of functional group = 73. Ester produced in 8 h =
III.
7.6 × 28.8 × 146
10
3
= 32 kg.
KINETICS OF CONDENSATION POLYMERIZATION
Flory’s equal reactivity principle, which has been validated on mechanistic and experimental grounds,
has greatly simplified an otherwise complicated kinetic analysis of condensation polymerization. This
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163
CONDENSATION (STEP-REACTION) POLYMERIZATION
principle in effect amounts kinetically to the proposition that all steps in a condensation polymerization
have equal rate constants. We now consider two cases.
Case 1: Polymerization without Added Strong Acid: Consider esterification — the formation of a
polyester from a glycol and a dibasic acid. The progress of reaction is easily followed by titrating the
unreacted carboxyl groups in samples removed from the reaction mixture. This polyesterification and
other simple esterifications are acid-catalyzed. In the absence of an added strong acid, a second molecule
of the acid being esterified acts as the
O
HO
C
O
O
(CH2)4
C
OH + HO
adipic acid
O
C (CH2)4 C
O
CH2CH2
O
CH2CH2
OH
diethylene glycol
CH2CH2
O
CH2CH2
O
+ 2n H2O
n
(Str. 2)
catalyst. The rate of polyesterification process can therefore be written:
− d [COOH]
2
= k [OH] [COOH]
dt
(6.1)
where concentrations (written in square brackets) are expressed as equivalents of the functional groups.
As written here Equation 6.1 assumes that the rate constant k is independent of molecular size of reacting
species and is the same for all functional groups.
If the concentration, C, of the unreacted carboxyl and hydroxyl groups at time t are equal, Equation 6.1
may be rewritten as:
− dc
= kc3
dt
(6.2)
On integration, this yields the third-order reaction expression
2 kt =
1
− constant
c2
(6.3)
Now, let us introduce the extent of reaction, p, defined as the fraction of the functional group that has
reacted at time t. That is,
p=
Co − C
Co
(6.4)
where Co = initial concentration of one of the reactants. From Equation 6.4,
C = C o (1 − p)
(6.5)
Hence, on substitution, Equation 6.3 becomes
2C o2 kt =
Copyright 2000 by CRC Press LLC
1
−1
(1 − p)2
(6.6)
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POLYMER SCIENCE AND TECHNOLOGY
Figure 6.1 Reaction of diethylene glycol with adipic acid (DE-A) and of diethylene glycol with caproic acid
(DE-C). Time values at 202°C have been multiplied by two. (From Flory, P.J., J. Am. Chem. Soc., 61, 3334,
1939; 62, 2261, 1940. Copyright 1940 American Chemical Society. With permission.)
A plot of
1
(1 − p)2
against time should be linear. Figure 6.1 verifies this third-order kinetic expression.
Note from Figure 6.1 that when
1
The quantity
1
1− p
= 25 (i.e., when
1
= xn = 5) the plot is not linear. However,
(1 − p)
1− p
this behavior is not unique to polyesterification. A similar behavior is exhibited by the nonpolymer
esterification of diethylene glycol and caproic acid. The similarity in behavior of both mono- and
polyesterification provides a direct verification of the nondependence of reactivity on molecular size.
2
is called the degree of polymerization, DP, which as we saw earlier corresponds to
the average number of monomer molecules in the chain.
Case II: Polymerization with Added Strong Acid: The kinetic expression can be greatly simplified
if the polyesterification is carried out in the presence of a small amount of strong acid, e.g., p-toluene
sulfonic acid. With the catalyst concentration kept constant throughout the process, the rate expression
becomes
− dc
= k ′c2
dt
(6.7)
where k′ = k [catalyst] and where the alcohol and carboxylic acid concentrations are kept constant.
Integrating this second-order rate equation and inserting the extent of reaction, p, we have:
C o k ′t =
Figure 6.2 shows the linear relation between
IV.
1
1− p
1
−1
1− p
(6.8)
and time.
STOICHIOMETRY IN LINEAR SYSTEMS
As noted earlier, linear polymers are obtained from condensation polymerization when the functionality
of the reactants is 2. Two cases may be considered. Some commercial examples of step-growth polymerization are illustrated in Figure 6.3. Reactions A and B are two routes (esterification and ester
interchange) for preparing the same compound [poly(ethylene terephthalate), PETP]. Reaction C is
Copyright 2000 by CRC Press LLC
CONDENSATION (STEP-REACTION) POLYMERIZATION
165
Figure 6.2 Reaction of decamethylene glycol with adipic acid at the temperature indicated, catalyzed by
0.10 eq% of p-toluenesulfonic acid. The time scale for the results at 82.8°C is to be multiplied by two. (From
Flory, P.J., J. Am. Chem. Soc., 61, 3334, 1939; 62, 2261, 1940. Copyright 1940 American Chemical Society.
With permission.)
preparation of nylon 6,6 from adipic acid and hexamethylenediamine. Note that in each of these three
reactions, each of the two monomers is bifunctional and contains the same functionality at each end,
that is, A–A or B–B. Alternatively, a polymer can also be formed by intramolecular reaction. An example
is the formation of an aliphatic polyester by the self-condensation of ω-hydroxycarboxylic acid (reaction D,
Figure 6.3). Since the end functional groups of the acid are different, this polyesterification reaction is
an example of an A-B step-growth condensation polymerization.
We now proceed to discuss the stoichiometry of these two types of linear system represented by:
I: A − B → (A − B− A − B− A − B) + small molecule
II: A − A + B− B → A(B− A − B− A − B− A) − B + simple molecule
(Str. 3)
Suppose we start with the bifunctional monomers exclusively such that the number of A groups is
precisely equivalent to that of B groups. Obviously, the requirement that A and B groups be present in
equivalent amounts is automatically satisfied by A–B type polycondensation provided the monomers are
pure and no side reactions occur. For type A–A/B–B polymerization we require, in addition, that the
reactants must be present in equivalent proportions. At any stage of the polymerization process, each
molecule will be terminated on either side by an unreacted functional group. Therefore, the number of
molecules present can be determined by measuring the number of structural units per molecule, i.e., the
number-average degree of polymerization, Xn, is given by the ratio of the initial number of molecules
to the final number of molecules, i.e.,
Xn =
=
Copyright 2000 by CRC Press LLC
original number of molecules
final number of molecules
Co
1
=
C o (1 − p) 1 − p
(6.9)
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POLYMER SCIENCE AND TECHNOLOGY
A.
n HO
O
O
C
C
OH + n HO
Terephthalic acid
CH2CH2
OH
Ethylene glycol
O
O
C
C
O
CH2CH2
O
+ 2n H2O
Poly (ethylene terephthalate)
B.
n H3C
O
O
O
C
C
O
CH3 + n HO
Dimethyl terephthalate
CH2CH2
Weak basic
catalyst
OH
Ethylene glycol
O
O
C
C
O
CH2CH2
O
+ 2n CH3 OH
n
Poly (ethylene terephthalate)
C.
C
n HO
C
C
(CH2)4
C
OH + n H2N
Adipic acid
(CH2)6
NH2
Hexamethylenediamine
O
C
O
(CH2)4
C
NH
(CH2)6
NH
+ 2n H2O
n
Nylon 6/6
D.
C
n HO
(CH2)5
O
C
C
OH
∆
(CH2)5
C
O
+ 2n H2O
n
ω-Hydroxycaproic acid
Polycaprolactone
Figure 6.3 Classical condensation reactions; are (A) esterification, (B) ester interchange, (C) amidization, and
(D) intramolecular reaction. Two routes, A and B, can be taken to prepare PET. (From Fried, J.R., Plast. Eng.,
38(10), 27, 1982. With permission.)
From the definition of Xn, it follows that the number-average molecular weight will be given by the
expression:
Mn = Xn Mo =
Copyright 2000 by CRC Press LLC
Mo
1− p
(6.10)
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CONDENSATION (STEP-REACTION) POLYMERIZATION
where Mo = average molecular weight of the structural unit.
To avoid confusion, it is necessary to reemphasize the definition of structural unit — the residue from
a glycol or from a dibasic acid. This means that the number of structural units equals the total number
of bifunctional monomers initially present. Thus the repeating unit of a chain derived from A–A/B–B
type monomers consists of two structural units, while the repeat unit in the case of type A–B monomers
is the same as the structural unit.
Table 6.1 High Molecular Weight Achieved Only by High Conversion
P
Mn
0
1
0.5
2
0.8
5
0.9
10
0.95
20
0.99
100
0.999
1,000
0.9999
10,000
1.0
∞
A closer look at Equation 6.9 reveals that in condensation polymerization, a high molecular weight
product is obtained only when the extent of reaction is almost 100% as shown in Table 6.1. Polymers,
by definition, derive their unique properties from their high molecular weights. Thus, commercially
useful condensation polymers are obtained only under the rather stringent condition of almost quantitative
reaction. This means that in addition to high conversion, a step-growth polymerization requires high
yield and high monomer(s) purity.
Example 6.2: For condensation polymerization between a dibasic acid and a glycol, show that Mo is
the average molecular weight of the structural units where the structural unit is the residue from each
monomer.
Solution: Consider the polycondensation reaction
O
nHOOC
CH2
COOH + nHO
CH2
HO
C
O
CH2
C
O
CH2
+ 2n H2O (Str. 4)
O
n
Assume that we start with 10 moles of each monomer and suppose that the extent of reaction at a time
t equals p = 0.5. By definition
p=
Ni − N
Ni
Ni = initial number of moles
N = number of moles at time t
At time t,
N = Ni (1 – p)
= 20 (1 – 0.5)
= 10
Now
Xn =
1
1
=
=2
1 − P 1 − 0 .5
Mn = Xn Mo =
Mo = molecular weight of structural unit
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Mo
= 2 Mo
1− P
168
POLYMER SCIENCE AND TECHNOLOGY
Three cases have to be considered.
Case 1:
Mo = molecular weight of repeating unit
= 116 kg/kg mol
Mn = 2Mo = 232 kg/kg mol
Mass of polymer = 232 × 10 = 2320 kg
Case 2:
Mo = average molecular weight of starting monomers
+ 48
--------------------- = 76
= 104
2
Mn = 2Mo = 152
Mass of polymer produced at t = 152 × 10 = 1520 kg
Case 3:
Mo = average molecular weight of structural units where structural unit equals residue from each
monomer
--------- = 58
= 116
2
Mn = 2 × 58 = 116
Mass of polymer produced = Mn × 10 = 1160 kg
To establish which of the above definitions of Mo is correct, we need to carry out a simple mass balance:
Initial mass = sum of masses of reactants = 104 × 10 + 48 × 10 = 1520 kg
Mass of product = mass of polymer + mass of H2O split off
Mass of water split off = 2nH2O = 2 × 10 × 18 = 360
Case 1: mass of product = 2320 + 360 = 2680 kg
Case 2: mass of product = 1520 + 360 = 1880 kg
Case 3: mass of product = 1160 + 360 = 1520 kg
From mass balance, it is obvious that only Case 3 gives a correct mass balance.
Example 6.3: A 21.3 g sample of poly(hexamethylene adipamide) is found to contain 2.50 × 10–3 mol
of carboxyl groups by both titration with base and infrared spectroscopy. From these data calculate
a. The number-average molecular weight
b. The extent of reaction
What assumption is made in your calculations?
H
N
(CH2)6
H
O
N
C
O
(CH2)4
C
Mn =
∑ Wi W
21.3
=
=
= 8520 g g mol
N 2.5 × 10 −3
∑ Ni
Xn =
M
1
= n
1 − p Mo
--------- = 113.
where Mo = average molecular weight of residue = 226
2
Copyright 2000 by CRC Press LLC
(MW = 226)
(Str. 5)
169
CONDENSATION (STEP-REACTION) POLYMERIZATION
1
8520
=
= 75.40
1− p
113
1 − p = 0.013
p = 98.7%
Assumption is that each polymer molecule contains one –COOH group, i.e.,
H2N
(CH2)6
H
O
N
C
(CH2)4
(Str. 6)
COOH
V. MOLECULAR WEIGHT CONTROL
From the above discussion, it is obvious that in addition to high conversion, a step-growth polymerization
requires high yield and high monomer purity to obtain a polymer of high molecular weight. High yield
means the absence of any side reactions that could deactivate the polymerization process. For example,
any side reactions that would lead to monofunctional units that are incapable of further reaction would
limit the formation of high molecular weight of A–A/B–B or A–B type polymers. In general, depression
of molecular weight can be brought about by:
• Nonequivalence of reactants
• Monofunctional ingredients introduced as impurities formed by side reaction
• Unbalance in stoichiometric proportions
Suppose an excess of a functional group is obtained by the addition of reactant designated B+B. In
this case the two types of polymerizations discussed above become:
Case I: A–B + little B+B
Case II: A–A + B– + little B+B
Then let
NA = total number of A groups initially present
NB = total number of B groups initially present
r=
NA
< 1 i.e., N A < N B
NB
(6.11)
PA = fraction of A groups that have reacted at a given stage of the reaction
Total number of units =
NA + NB
=
2
NA
2
1 + 1
r
(6.12)
At any stage of the reaction, possible types of chains are
A–BA–BA–BA–B
B–AB–AB–AB–AB–B
(type IA)
(type IB)
Number of molecules of type IA = NA (1 – PA), and number of molecules of type IB =
number of molecules due to excess reactant):
Copyright 2000 by CRC Press LLC
NB − NA
2
(i.e.,
170
POLYMER SCIENCE AND TECHNOLOGY
Total number of molecules at any stage of the reaction:
N A (1 − p A ) +
=
NA 1
−1
2 r
[
]
NA
r 2 r (1 − PA ) + ( r − r )
2
Now
Xn =
original no. of molecules
final no. of molecules
(6.13)
1+ r
2 r (1 − PA ) + 1 − r
(6.14)
Xn =
For r = 1 (for stoichiometric amounts of A and B)
Xn =
1
1− p
(6.15)
For p = 1, i.e., degree of polymerization is maximum.
Xn =
1+ r
1− r
(6.16)
By playing around with Equation 6.14 we see how the degree of polymerization and hence the molecular
weight of the product is influenced by a proper control of the purity of reactants and prevention of
extraneous reactions. In practice the molecular weight of nylons can be stabilized by the deliberate
addition of a predetermined amount of a monofunctional monomer like acetic acid.
Note that Equation 6.14 can be used to evaluate the degree of polymerization in a system containing
bifunctional reactants and a small amount of monofunctional species provided r is defined as
r=
VI.
NA
N A + 2 N B+
(6.17)
MOLECULAR WEIGHT DISTRIBUTION IN LINEAR
CONDENSATION SYSTEMS
From Flory’s equal reactivity principle, it follows that each functional group during condensation
polymerization has an equal chance of reacting irrespective of the molecular size of the group to which
it is attached. Therefore, the probability that a given functional group has reacted should simply be equal
to the extent of reaction of that functional group. For type A–B polycondensations or for stoichiometric
quantities in type A–A/B–B polymerization, this probability is equal to PA = PB. Obviously, since the
reaction of functional groups is a random event, various molecular sizes of product will be present in
the reaction mixture at any particular stage of the reaction. The question therefore is the probability of
finding a molecule of size, say, X. Consider the following molecule of A–B type polycondensation:
ARB1 − ARB2 − ARB3 ………… ARB− ARBx −1 − ARBx
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(Str. 7)
171
CONDENSATION (STEP-REACTION) POLYMERIZATION
The probability that the first functional group B has undergone condensation with A is p, the extent of
reaction. By similar reasoning, the probability that the second B has also reacted is also p and so on. A
molecule containing X units must have undergone X – 1 reactions. The probability that this number of
reactions has occurred is simply the product of individual reaction probabilities, i.e., px–1. The probability
of finding an unreacted end group is 1 – p. Therefore the total probability, Px, that a given polymer
molecule contains X units is
Px = p x −1 (1 − p) .
(6.18)
Evidently this Px must be equal to the mole fraction, nx, of X-mers in the reaction system of the extent
of reaction p.
Px = n x =
Nx
N
(6.19)
where N is the total number of molecules of all sizes in the system and Nx is the number of X-mers.
But N is related to the initial number of monomers, No, by the relation:
N = N o (1 − p)
(6.20)
On substitution the number of X-mers in terms of the initial amount of monomers and the extent of
reaction, Equation 6.19 becomes:
N x = N o p x −1 (1 − p)
2
(6.21)
The weight fraction, Wx, of X-mers is obtained by dividing the weight of X-mers by the weight of
all the molecules, i.e.,
Wx =
xN x
No
(6.22)
or substitution, in Equation 6.21, we obtain
Wx = xp x −1 (1 − p) .
2
(6.23)
The distributions given in Equations 6.21 and 6.23 are known as the most probable distributions and
are shown graphically in Figures 6.4 and 6.5.
From Equation 6.21 it is observed that on a mole fraction basis, low-molecular-weight chains are
most abundant even at high extents of reaction. However, on a weight fraction basis (Equation 6.23),
Wx passes through a maximum near the number average value of x. Also, low-molecular-weight chains
are less significant.
VII.
MOLECULAR WEIGHT AVERAGES
The most probable distributions derived above lead to expressions for molecular weight averages Mn
and Mw . From Chapter 3, Mn is defined by the relation:
∞
∞
Mn =
∑
MxNx
∑N
x =1
Copyright 2000 by CRC Press LLC
x
∑ (M X) N
o
=
x −1
∞
x =1
∞
∑N
x =1
x
x
(6.24)
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POLYMER SCIENCE AND TECHNOLOGY
Figure 6.4 Mole fraction distribution of chain molecules in a linear condensation polymer for several extents
of reaction. (From Flory, P.J., Chem. Rev., 39, 137, 1946. With permission.)
Figure 6.5 Weight fraction distribution of chain molecules in linear condensation polymers for several extents
of reaction. (From Flory, P.J., Chem. Rev., 39, 137, 1946. With permission.)
Here Mn = molecular weight of an X-mer. Thus,
M n = M o ∑ x (1 − p) p x −1
(6.25)
X n = ∑ x (1 − p) p x −1
(6.26)
It can be shown that
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173
CONDENSATION (STEP-REACTION) POLYMERIZATION
Xn =
1
1− p
(6.27)
Mn =
Mo
1− p
(6.28)
and
The weight-average molecular weight is defined by:
∞
∞
Mw =
∑
o
=
x −1
∞
∑
∑ M (X) N
2
Mxwx
wx
x =1
∞
∑
x
(6.29)
M o (X) N x
x =1
x =1
where wx = weight fraction of X-mers
w
weight of X-mers
= x
w weight of all molecules
From where
Xw =
1+ p
1− p
(6.30)
and
Mw = M o X w
(6.31)
Xn
= 1+ p
Xn
(6.32)
The ratio
is a quantitative measure of polydispersity or the spread of the molecular weight curve as
p → 1,
Xw
→2
Xn
(6.33)
Example 6.4: a. Polyester fibers for “lace” material can, in principle, be produced from ω = hydroxycaproic acid. If the initial 100 moles of the hydroxyacid are reduced to 2 moles after 10 h reaction
time, calculate:
1.
2.
3.
4.
Copyright 2000 by CRC Press LLC
The
The
The
The
number average molecular weight Mn
weight average molecular weight Mw
probability that the reaction mixture contains tetramers
weight fraction of these tetramers.
174
POLYMER SCIENCE AND TECHNOLOGY
b. As a result of extraneous reactions of the hydroxyl groups, a 5% excess of the carboxylic acid
is present in the reaction mixture. Calculate the number-average molecular weight for the same
extent of reaction in a.
Solutions: a. The polymerization of ω-hydroxycaproic acid proceeds according to the following equation:
O
nHO
(CH2)5
C
O
OH
∆
Extent of reaction p =
(CH2)5
(Str. 8)
n
Mo
114
=
= 5700
1 − p 0.02
Mn = Xn Mo +
2.
1 + p
1.98
Mw = X w M o = M o
= 114 0.02 = 11, 286
p
−
1
3.
Px = p x −1 (1 − p)
4.
+ 2nH2O
O
C o − C 100 − 2
=
= 0.98
100
Co
1.
= (0.98)
C
(1 − 0.98) = 0.019
4 −1
( )
Wx = X p x −1 (1 − p)
= 4 (0.98) (0.02) = 1.51 × 10 −3
3
b.
Xn =
r=
Xn =
2
1+ r
2 r (1 − p) + 1 − r
100
= 0.95
105
1 + 0.95
= 22.16
2 (0.95) (1 − 0.98) + 1 − 0.95
M n = X n M o = 2526
Notice the depression in Mn as a result of stoichiometric imbalance.
VIII.
RING FORMATION VS. CHAIN POLYMERIZATION
We observed earlier that polymer formation requires that the reacting monomer(s) must be at least
bifunctional. It is pertinent to point out at this stage that polyfunctionality of reactants is a necessary
but not sufficient condition for the formation of a polymer. Bifunctional monomers whether of the
A–A/B–B or A–B types may react intramolecularly to produce cyclic products. For example, hydroxy
acids when heated may yield lactams or linear polyamides.
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175
CONDENSATION (STEP-REACTION) POLYMERIZATION
C
O
R
O
lactam
HORCOOH
O
H
O
R
C
OH
x
polyamide
(Str. 9)
The prime factor that determines the type of product is the size of the ring that is obtained by cyclization.
If the ring size is less than five atoms or more than seven, polymer formation is almost entirely favored.
If a five-membered ring can be formed, this occurs exclusively. If a ring of six or seven atoms can form,
either the ring or linear polymer or both will be the product. Larger rings can be formed under special
conditions.
The difficulty of forming rings with less than five members is due to the strain imposed by the valence
angles of the ring atom. Five-membered rings are strain-free while all larger rings can be strainless if
nonplanar forms are possible.
IX.
THREE-DIMENSIONAL NETWORK STEP-REACTION POLYMERS
As recalled from our earlier discussion, we stated that if one of the reactants in step-growth polymerization has a functionality greater than 2, then the formation of a branched or a three-dimensional or
cross-linked polymer is potentially possible. An example is the reaction between a dibasic acid and
glycerol, which has two primary and one secondary hydroxyl groups (functionality of 3). A monomer
that possesses such a functionality is referred to as a branch unit. Every reaction of such a molecule
introduces a branch point for the development of the three-dimensional network. The portion of a polymer
molecule lying between two branch points or a branch point and a chain end is called a chain section
or segment.
As polymerization proceeds many branch points are formed. Reaction between large molecules
considerably increases the number of reaction groups per polymer chain. The size of the polymer
molecules increases rapidly, culminating in the formation of a three-dimensional network polymer of
infinite molecular weight. During this process, the viscosity of the reaction medium increases gradually
at first and experiences a sudden and enormous increase just before the formation of the three-dimensional
network (Figure 6.6). The reaction medium loses fluidity, and bubbles cease to rise through it. Gelation
is said to have occurred at this point, which is referred to as the gel point.
The sudden onset of gelation does not indicate that all the reactants have become bound together in
the resulting three-dimensional network. Gelation marks the division of the reaction medium into an
insoluble material called the gel and a portion still soluble in an appropriate solvent referred to as the
sol. If polymerization is continued beyond the gel point, the gel fraction increases at the expense of the
sol. As would be expected, at gelation there is a considerable depletion of individual molecules, and
consequently the number-average molecular weight becomes very low. On the other hand, the weightaverage molecular weight becomes infinite.
X.
PREDICTION OF THE GEL POINT
We want to be able to define under which conditions a three-dimensional network will be formed, that
is, the point at which gelation takes place during a reaction. To do this, let us introduce the term branching
coefficient, α, which is defined as the probability that a given functional group of a branch unit is
connected via a chain of bifunctional units to another branch unit. Let us illustrate this through the
following example.
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POLYMER SCIENCE AND TECHNOLOGY
Figure 6.6 The course of typical three-dimensional polyesterification. (From Flory, P.J., J. Am. Chem. Soc., 63,
3083, 1941. Copyright 1941 American Chemical Society.)
(Str. 10)
where i varies from 0 to ∞.
In considering this reaction, assumptions are made:
• The principle of equal reactivity of functional groups holds through the condensation. This means that
the reactivity of a given A or B group is independent of the size or structure to which the group is
attached. In practice this is not strictly true, for it is known, for example, that in glycerol the secondary
hydroxyl is less reactive than either of the two primary groups.
• Intramolecular reactions between A and B are forbidden. This, again, is not entirely correct.
We will, however, neglect for the moment errors that will be introduced into our calculations as a result
of these assumptions. With these assumptions in mind, we note that the probability that A* has reacted
is PA, the fraction (or extent of reaction) of A groups that have reacted. Similarly, the probability that
B** has reacted is PB .
Now let us define another fractional parameter ρ, as the ratio of A groups both reacted and unreacted
on the branch units to all or the total A groups in the mixture. In this case, the probability that a B group
has reacted with an A on a branch unit is P Bρ while the probability that a B group has reacted with an
A group on a bifunctional unit (A–A) is PB (1 – ρ). It follows that the probability that the chain segment
shown is formed is given by PA *PB(1 – ρ) PA * i PB ρ. Summing overall values of i yields
α=
If we define r =
then PA = rPA.
Copyright 2000 by CRC Press LLC
NA
NB
=
A groups initially present
B groups initially present
,
PA PB ρ
1 − PA PB (1 − ρ)
(6.34)
CONDENSATION (STEP-REACTION) POLYMERIZATION
177
To eliminate either PA or PB, we substitute this relation in Equation 6.34 to obtain:
α=
rPA2 ρ
1 − rPA2 (1 − ρ)
(6.35)
α=
PB2 ρ
r − PB2 (1 − ρ)
(6.36)
or
Consider special cases of interest.
1. When A and B groups are present in equivalent quantities: r = 1, PA = PB = P, and
α=
P 2ρ
1 − P 2 (1 − ρ)
(6.37)
2. If there are no bifunctional A–R–A units, ρ = 1 and
α = rPA2 = PB2 r
(6.38)
3. If the above two conditions apply, r = ρ = 1 and
α = p2
(6.39)
4. If there are only branch units, then the probability that a functional group on a branch units leads to
another branch unit is simply the extent of reaction
α=p
(6.40)
Note that the above equations are completely general and independent of the functionality of the branch
unit.
The outstanding problem is to deduce the critical value of α at which the formation of an infinite
network becomes possible. To tackle this problem, we consider a branch unit which is trifunctional. In
this case, each chain that terminates in a branch unit is succeeded by two more chains. These two chains
will, in turn, generate four additional chains and the propagation of chains will continue in like manner.
In general therefore for trifunctional branch units, chain sections emanating from n chains will generate
2n chains under the conditions described. Recall, however, that the probability that chain segments
originating from branch units will terminate in other branch units is α. Therefore the expected number
of branch units from n branch units is 2nα. Now, it is easy to visualize that the condition for continued
expansion of the network is that the number of chains emanating from n chains must be greater than n.
That is, the succeeding generation of chains must outnumber the preceding generation. For a trifunctional
unit, therefore, if α < 1/2 (i.e., 2nα < n), an infinite network cannot be generated since there is less than
an even chance that each chain will terminate in a branch unit. On the hand, if α > 1/2 (i.e., 2nα > n),
generation of an infinite network is possible. It is obvious that for a trifunctional branch unit, the critical
value of α is α = 1/2. This argument can be extended to systems containing polyfunctional branch units
with the general result that
αc =
Copyright 2000 by CRC Press LLC
1
f −1
(6.41)
178
POLYMER SCIENCE AND TECHNOLOGY
where f is the functionality of the branch unit. If there is more than one type of branch unit present,
(f – 1) must be replaced by an appropriate average weighted according to the number of functional
groups attached to the various branched units and the molar amount of each present.
XI.
MORPHOLOGY OF CROSS-LINKED POLYMERS
As we saw in Section 3, the properties of polymeric materials are dictated by their morphologies.
Considerable knowledge has been accumulated about the morphology of semicrystalline polymers over
the last four to five decades. Unfortunately, the same cannot be said for cross-linked polymers or
thermosets. From the discussion in Section IX for example, in the absence of intramolecular reactions,
polyfunctional condensation reactions where different functional groups are chemically equivalent result
in a giant molecule with an infinite cross-linked network. For a long time, this concept of an infinitely
large molecule with a homogeneous cross-linked network was used to describe the morphology of crosslinked polymers. However, the presence of inhomogeneities in the cross-link density of polymer networks
has been established empirically through electron microscopy, thermomechanical studies, and swelling
experiments. Today, it is generally recognized that the model that most adequately describes the morphology of thermosets is that of an inhomogeneously cross-linked network consisting of regions of
higher cross-linked density (referred to as nodules) immersed in a less cross-linked matrix.5,6
XII.
PROBLEMS
6.1. United Nigeria Textile (Ltd) UNTL Kaduna is making nylon 6,6 from hexamethylenediamine and adipic
acid. One batch is made on every 8-h shift. In each batch equimolar reactants are used and conversion
is usually 98.0%. At the end of the run the bulk product is extruded and chopped into pellets.
a. Calculate the number-average molecular weight.
b. The afternoon shift operator dumped in too much adipic acid. From his records you calculate that
the mole ratio was 2% excess adipic acid. If the batch went to the usual conversion, what was its
number-average molecular weight?
c. The night shift operator weighed things correctly but fell asleep and let the reaction run too long —
99% conversion. What will be the Mn of this batch?
d. How should the UNTL engineer mix these batches to obtain the Mn of the usual product. What will
be Mw?
6.2. A polyester system has a number-average molecular weight Mn of 6000 and a polydispersity of 2. The
system is fractionated into two samples of the Mn (2000 and 10,000, respectively). Equimolar proportions
of these are mixed.
a. What are the Mn and Mw for the new (mixed) system?
b. How will the melt viscosity of the new system (fractionated and mixed) change in relation to the
unfractionated (original) polymer?
6.3. A polymer chemist prepared nylon from the following amino acid (ω-amino caproic acid): [NH2 – (CH2)5 –
COOH]. Due to improper purification of the reactant, side reactions occurred leading to a 5% stoichiometric imbalance (i.e., 5% excess of one of the functional groups). Calculate:
a. The number-average molecular weight if conversion was 98%.
b. The number-average molecular weight for the maximum degree of polymerization.
6.4. Consider a polyesterification reaction.
a. Suppose 1 mol each of dicarboxylic acid and glycol is used. What is the degree of polymerization
when the extent of reaction is 0.5, 0.99, and 1.0?
b. Suppose 101 mol glycol is reached with 100 mol of the dicarboxylic acid. What is the maximum
degree of polymerization?
c. Suppose the dicarboxylic acid contains 2 mol% monoacid impurity. What is the maximum degree of
polymerization?
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179
CONDENSATION (STEP-REACTION) POLYMERIZATION
6.5. The preparation of poly(ethylene terephthalate) from terephthalic acid and ethylene glycol is stopped at
99% conversion. Calculate:
a.
b.
c.
d.
The
The
The
The
number-average degree of polymerization.
weight-average degree of polymerization.
probability that the reaction mixture contains trimers.
weight-fraction of the trimers.
6.6. The preparation of nylon 12 is shown by the following equation.
H
H2N
(CH2)11
COOH
O
(CH2)11
N
(Str. 11)
C
n
Calculate the weight fraction of n-mers in the reaction mixture if the conversion is
a. 10%
b. 90%
for n = 1 and 100. Comment on your answer.
6.7. What are the number-average molecular weights for the following nylons if 95% of the functional groups
have reacted?
O
H
C
(CH2)5
O
C
(CH2)4
O
H
C
N
N
100
(Str. 12)
nylon 6
H
(CH2)12
N
nylon 6,12
(Str. 13)
100
What is the weight fraction of monomers in the mixture?
6.8. Calculate (1) the mole fraction and (2) the weight fraction of the following polymer in the reaction
medium if 98% of the functional groups has undergone conversion.
CO
(CH2)5
CO
NH
(CH2)9
(Str. 14)
NH
100
What is the weight fraction of the monomers in the mixture?
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
Flory, P.J., J. Am Chem. Soc., 61, 3334, 1939; 62, 2261, 1940.
Fried, J.R., Plast. Eng., 38(10), 27, 1982.
Flory, P.J., Chem. Rev., 39, 137, 1946.
Flory, P.J., J. Am. Chem. Soc., 63, 3083, 1941.
Kreibich, U.T. and Schmid, R., J. Polym. Sci. Symp., 53, 177, 1975.
Mijovic, J. and Lin, K., J. Appl. Polym. Sci., 32, 3211, 1986.
Mijovic, J. and Koutsky, J.A., Polymer, 20(9), 1095, 1979.
Flory, P.J., Principles of Polymer Chemistry, Cornell University Press, Ithaca, NY, 1953.
Billmeyer, F.W., Jr., Textbook of Polymer Science, 2nd ed., Interscience, New York, 1971.
Kaufman, H.S. and Falcetta, J.J., Introduction to Polymer Science and Technology, Interscience, New York, 1977.
Rudin, A., The Elements of Polymer Science and Engineering, Academic Press, New York, 1982.
Williams, D.J., Polymer Science and Engineering, Prentice-Hall, Englewood Cliffs, NJ, 1971.
Copyright 2000 by CRC Press LLC
Chapter 7
Chain-Reaction (Addition) Polymerization
I.
INTRODUCTION
Unsaturated monomers are converted to polymers through chain reactions. In chain polymerization
processes, the active center is retained at the end of a growing polymer chain and monomers are added
to this center in rapid succession. The rate of addition of monomers to the active center relative to the
overall conversion of the monomer to polymer is quite fast. This means that high-molecular-weight
polymers are generated even while most of the monomers remain intact. Also, polymers formed at the
initial stage of the reaction are little affected by the extent of reaction and are of comparable molecular
weight with those formed later in the reaction. Thus in a partially polymerized mixture, only highmolecular-weight polymers and unchanged monomers are present with virtually no intermediates
between these ends of the molecular weight spectrum.
II.
VINYL MONOMERS
From elementary organic chemistry, we know that the positions and hence reactivities of the electrons
in unsaturated molecules are influenced by the nature, number, and spatial arrangement of the substituents
on the double bond. As a result of these influences, the double bond reacts well with a free radical for
compounds of the types CH2 = CHY and CH2 = CXY. These compounds constitute the so-called vinyl
monomers where X and Y may be halogen, alkyl, ester, phenyl, or other groups. It must, however, be
noted that not all vinyl monomers produce high polymers. In symmetrically disubstituted double bonds
(e.g., 1, 2 disubstituted ethylenes) and sterically hindered compounds of the type CH2 = CXY, polymerization, if it occurs at all, proceeds slowly.
The active centers that are responsible for the conversion of monomer to polymer are usually present
in very low concentrations and may be free radicals, carbanions, carbonium ions, or coordination
complexes among the growing chain, the catalyst surface, and the adding monomer. Based on the nature
of the active center, chain-reaction polymerization may be subdivided into free-radical, ionic (anionic
or cationic), or coordination polymerization. Whether or not the polymerization of a particular monomer
proceeds through one or more of these active centers depends partly on the chemical nature of the
substituent group(s). Monomers with an electron-withdrawing group can proceed by an anionic pathway;
those with an electron-donating group by a cationic pathway. Some monomers with a resonance-stabilized
substituent group — for example, styrene — can be polymerized by more than one mechanism. Table 7.1
summarizes the various polymerization mechanisms for several monomers.
III.
MECHANISM OF CHAIN POLYMERIZATION
Irrespective of the character of the active center, chain-reaction polymerization, like all chain reactions,
consists of three fundamental steps. These are initiation, which involves the acquisition of the active
site by the monomer; propagation, which is the rapid and progressive addition of monomer to the growing
chain without a change in the number of active centers; and termination, which involves the destruction
of the growth activity of the chain leaving the polymer molecule(s). In addition to the above three
processes, there is the possibility of another process known as chain transfer during which the growth
activity is transferred from an active chain to a previously inactive species. We now discuss each of
these steps in greater detail.
A. INITIATION
Initiation, as noted above, involves the generation of active species, which can be through free-radical,
ionic, or coordination mechanism. This mechanism of chain initiation is the essential distinguishing feature
between different polymerization processes. Quite expectedly, therefore, addition polymerization of vinyl
0-8493-????-?/97/$0.00+$.50
© 1997 by CRC Press LLC
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182
POLYMER SCIENCE AND TECHNOLOGY
Table 7.1 Initiation Mechanisms Suitable for the Polymerization of Olefin Monomers
Olefin Monomer
Monomer Structure
Free Radical
Anionic
Cationic
Coordination
Ethylene
Propene
Butene-1
Isobutene
Butadiene-1,3
Isoprene
Styrene
Vinyl chloride
Vinylidene chloride
Vinyl fluoride
Tetrafluoroethylene
Vinyl ethers
Vinyl esters
Acrylic esters
Methacrylic esters
Arcrylonitrile
CH2› CH2
CH2› CHMe
CH2› CHEt
CH2› Cme2
CH2› CH–CH› CH2
CH2› C(Me)–CH–CH2
CH2–CHPh
CH2› CHCl
CH2› CCl2
CH2› CHF
CF2› CF2
CH2› CHOR
CH2› CHOCOR
CH2› CHCOOR
CH2› C(Me)COOR
CH2› CHCN
+
–
–
–
+
+
+
+
+
+
+
–
+
+
+
+
–
–
–
–
+
+
+
–
+
–
–
–
–
+
+
+
+
–
–
+
–
–
+
–
–
–
–
+
–
–
–
–
+
+
+
–
+
+
+
+
–
–
+
+
–
+
+
+
Note: +, monomer can be polymerized to high-molecular-weight polymer by this form of initiation; –, no
polymerization reaction occurs or only low-molecular-weight polymers or oligomers are obtained with
this type of initiator.
From Lenz, R.W., Organic Chemistry of High Polymers, Interscience, New York, 1967. © John Wiley & Sons.
Reprinted with permission of John Wiley & Sons.
monomers differs from system to system, and a proper choice of the nature of the system vis-a-vis the
monomer being polymerized is crucial to the degree of success of polymerization of vinyl monomers.
1. Generation of Free Radicals
Free radicals can be generated by a number of ways, including thermal or photochemical decomposition
of organic peroxides, hydroperoxides, or azo or diazo compounds. Other methods of generation of free
radicals include dissociation of covalent bonds by high-energy irradiation and oxidation–reduction
(redox) reactions. The active species produced by these processes are referred to as initiators. These
species are frequently but erroneously also called catalysts. Initiators are consumed in the reaction while
catalysts are regenerated after the reaction. Compounds usually used for free-radical generation include
• Benzoyl peroxide
O
C
O
O
O
C
C
O
O
2
•
2
• Azo-bis-isobutyronitrile (AIBN)
(CH3)2
CN
CN
NC (CH3)2
(CH3)2 C • + N2
CN
CN
• Potassium persulfate
K2 S2 O8
Copyright 2000 by CRC Press LLC
2K+ + 2SO4- •
+ 2CO2
(Str. 1)
183
CHAIN-REACTION (ADDITION) POLYMERIZATION
• t-Butylhydroperoxide
CH3
CH3
C
CH3
O
OH
CH3
CH3
C
(Str. 2)
O • + HO •
CH3
Peroxide-type initiators in aqueous system can be decomposed in redox reactions, particularly with
reducing agents like ferrous ions, to produce free radicals.
H 2O2 + Fe 2 + → Fe3+ + OH − + HO ⋅
(Str. 3)
S2O82− + Fe 2 + → Fe3+ + SO 42− + SO 4− ⋅
The initiation step in free-radical polymerization in the presence of thermal initiators is a two-step
process: first is the homolysis of the relatively weak covalent bond in the initiator, designated here by R:R
R: R → 2 R ⋅
(7.1)
Second, in the presence of a vinyl monomer, the free radical thus generated adds to one of the electrons
of one of the bonds constituting the double bonds in the monomer. The remaining electron now becomes
the new free radicals:
H
R•
H
CH2 • • C
+
R
CH2
X
(7.2)
C•
X
The first step is the rate-limiting step in the initiation process. That the above scheme represents that
initiation mechanism, and is indeed partly evidence for the free-radical mechanism in addition polymerization of vinyl monomers, has been established by the presence of initiator fragments found to be
covalently bound to polymer of molecules. The efficiency with which radicals initiate chains can be
estimated by comparing the amount of initiator decomposed with the number of polymer chains formed.
B. PROPAGATION
The free radical generated in the initiation step adds monomers in very rapid succession. Again, this
consists of an attack of the free radical on one of the carbon atoms in the double bond. One of the
electrons from the double bond pairs up with the free-radical electron forming a bond between the
initiator fragment and the attacked carbon atom. The remaining electron shifts to the “unattacked” carbon
atom of the double bond, which consequently becomes the free radical. In this way the active center is
transferred uniquely to the newly added monomer. This process then continues until termination sets in.
H
R
(CH2
CHX)n
CH2
C • + CH2
X
H
• •
C
X
H
R
(CH2
CHX)n+1
CH2
C•
(7.3)
X
For diene monomers, the propagation mechanism is exactly the same as that of vinyl monomers. But
the presence of two double bonds means either of two possible attacks can occur. Consequently,
propagation of dienes may proceed in one of two ways: 1,4 (trans or cis) and 1,2:
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184
POLYMER SCIENCE AND TECHNOLOGY
1,2 addition
CH2
X
X
C • + CH2
C
X
C
CH2
CH2
X
C
H
CH2
(7.4)
C•
H
CH
CH2
1,4 addition
CH2
X
X
C • + CH2
C
X
CH
CH2
CH2
H
C
X
CH2
C
H
CH
H
(7.5)
C•
H
In 1,2 addition, the unsaturation is part of the pendant group. In 1,4 addition, however, the unsaturation
is part of the backbone. Two configurations are possible — trans or cis. These considerations have
important consequences for the properties and hence end use of the resulting polymers.
C. TERMINATION
The propagation step would theoretically have continued until the consumption of all available monomers
but for the tendency of pairs of free radicals to react and annihilate their mutual activities. The termination
steps can occur by either of two mechanisms: combination (coupling) or disproportionation.
1. Combination or coupling — Two growing polymer chains yield a single polymer molecule terminated
at each end by an initiator fragment.
H
R
CH2
H
C• +
X
•C
CH2
R´
R
CH2
X
H
H
C
C
X
X
CH2
R´
(7.6)
2. Disproportionation — This involves hydrogen transfer with the formation of two polymer molecules,
one with a saturated end and the other with an unsaturated terminal olefin and each with an initiator
fragment.
H
R
CH2
C• +
X
H
•C
CH2
R´
R
CH2
X
H
H
C
H + C
X
X
CH
R´ (7.7)
D. CHAIN TRANSFER
In addition to the three fundamental processes described above for chain-growth (addition) polymerization, another important process, chain transfer, may occur. Chain transfer involves the transfer of radical
reactivity to another species, which may be a monomer, a polymer, a solvent, an initiator, or an impurity.
This creates a new species that is capable of further propagation but terminates the growth of the original
chain. The transfer reaction involves the transfer of an atom between the radical and the molecule. For
a saturated molecule like a solvent an atom is transferred to the radical:
H
R
CH2
C • + CCl4
X
Copyright 2000 by CRC Press LLC
H
R
CH2
C
X
Cl + CCl3•
(7.8)
185
CHAIN-REACTION (ADDITION) POLYMERIZATION
Figure 7.1 The influence of retarders and inhibitors on conversion.
On the other hand, if the molecule is unsaturated, like a monomer, the atom can be transferred to either
the monomer or the chain radical:
X
R
H
R
CH2
C • + CH2
CH2
CH2X + CH2
C•
CHX
(7.9)
H
H
C + CH3
C•
X
X
X
R
CH
It can be seen from this scheme that in chain transfer, radicals are neither created nor destroyed.
Consequently, the overall polymerization rate is unaffected by chain-transfer processes. However, as we
shall see presently, it does limit the obtainable molecular weight.
Chain transfer generally involves the cleavage of the weakest bond in the molecule. Consequently,
molecules with active hydrogens play important roles in chain polymerization. A material deliberately
added to the system to control molecular weight by chain transfer is called a chain-transfer agent. A
substance that, if added to the monomer, reacts with chain radicals to produce either nonradical products
or radicals with such low reactivity as to be incapable of adding monomer (and thus reducing the
polymerization rate) is called a retarder. If a retarder is so effective as to completely suppress the rate
and degree of polymerization, it is referred to as an inhibitor. Thus the distinction between the retarder
and inhibitor is merely that of degree. If an inhibitor added to a system is consumed by the radicals
generated within the system, an induction period during which no polymerization takes place ensues.
Thereafter, normal polymerization occurs. In some cases, added substances first inhibit the polymerization process and then allow polymerization to occur at slower than normal rates. These phenomena are
illustrated in Figure 7.1.
Retarders and inhibitors generally operate by chain-transfer mechanism. These substances may be
impurities in the system or are formed by side reactions during polymerization. In some cases, they are
added deliberately to the system to prevent inadvertent polymerization during transportation or storage.
A typical example is hydroquinone, normally added to styrene must be subsequently stripped of the
inhibitor before polymerization.
IV.
STEADY-STATE KINETICS OF FREE-RADICAL POLYMERIZATION
The overall mechanism for the conversion of a monomer to a polymer via free-radical initiation may be
described by rate equations according to the following scheme.
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POLYMER SCIENCE AND TECHNOLOGY
A. INITIATION
As noted earlier, this is a two-step process involving:
1. The decomposition of the initiator into primary radicals
kd
I
2R •
(7.10)
2. The addition of a monomer to the primary free radical
ka
R• + M
RMi•
(7.11)
The constants kd and ka are the rate constants for initiator dissociation and monomer addition, respectively.
Since initiator dissociation (eqn. 7.10) is much slower than monomer addition (eqn. 7.11), the first step
of the initiation step (initiator dissociation) is the rate-limiting step. Some of the initiator radicals may
undergo side (secondary) reactions, such as combination with another radical, that preclude monomer
addition. Therefore only a fraction, f (an efficiency factor), of the initial initiator concentration is effective
in the polymerization process. Also, decomposition of each initiator molecule produces a pair of free
radicals, either or both of which can initiate polymerization. Based on these observations, the rate
expression for initiation may be written as:
Ri =
d [M ⋅ ]
= 2 fk d [I]
dt
(7.12)
where [I] represents the initiator concentration.
B. PROPAGATION
The successive addition of monomers during propagation may be represented as follows:
k
RM1⋅ + M p → RM 2 ⋅
(7.13)
k
RM 2 ⋅ + M p → RM3⋅
In general,
k
RM x ⋅ + M p → RM x∑+1
(7.14)
The above scheme is based on the assumption that radical reactivity is independent of chain length.
Essentially, this means that all the propagation steps have the same rate constants kp. In addition,
propagation is a fast process. For example, under typical reaction conditions, a polymer of molecular
weight of about 107 may be produced in 0.1 s. It may therefore be assumed that the number of monomer
molecules reacting in the second initiation step is insignificant compared with that consumed in the
propagation step. Thus the rate of polymerization equals essentially the rate of consumption of monomers
in the propagation step. The rate expression for polymerization rate can therefore be written thus:
Rp = −
d [M]
= k p [M *] [M]
dt
(7.15)
where [M*] = ∑ RMx • i.e., the sum of the concentrations of all chain radicals of type RMx.
C. TERMINATION
Chain growth may be terminated at any point during polymerization by either or both of two mechanisms:
• Combination (coupling)
k tc
M x⋅ + M y ⋅
→ M x+y
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(7.16)
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CHAIN-REACTION (ADDITION) POLYMERIZATION
• Disproportionation
td
M x⋅ + M y ⋅
→ Mx + My
k
(7.17)
If there is no need to distinguish between the two types of termination, which in any case are kinetically
equivalent, then termination may be represented as:
kt
M x⋅ + M y ⋅
→P
(7.18)
where kt(ktc + ktd), ktc, and ktd are the rate constants for overall termination process, termination by
coupling, and termination by disproportionation, respectively. The termination rate is given by:
Rt =
− d [M ⋅]
2
= 2 k t [M ⋅]
dt
(7.19)
The factor of 2 arises from the fact that at each incidence of termination reaction, two radicals disappear.
Over the course of polymerization (at steady state), the total radical concentration remains constant.
This means that radicals are being produced and destroyed at equal rates (i.e., Ri = Rt). It follows from
Equations 7.12 and 7.19 that
fk d
kt
12
[M ⋅] =
[I] 1 2
(7.20)
Since the overall polymerization rate is essentially the rate of monomer consumption during propagation,
substitution of Equation 7.20 into Equation 7.15 yields:
fk
Rp = kp d
kt
12
[I] 1 2 [M]
(7.21)
Note that Equation 7.21 predicts that rate of polymer formation in free-radical polymerization is first
order in monomer concentration and half order in initiator concentration. This assumes, of course, that
the initiator efficiency is independent of monomer concentration. This is not strictly valid. In fact, in
practice Equation 7.21 is valid only at the initial stage of reaction; its validity beyond 10 to 15% requires
experimental verification. Abundant experimental evidence has confirmed the predicted proportionality
between the rate of polymerization and the square root of initiator concentration at low extents of reaction
(Figure 7.2). If the initiator efficiency, f, is independent of the monomer concentration, then Equation 7.21
predicts that the quantity Rp /[I]1/2 [M] should be constant. In several instances, this ratio has indeed been
found to show only a small decrease even over a wide range of dilution, indicating an initiator efficiency
that is independent of dilution. This confirmation of first-order kinetics with respect to the monomer
concentration suggests an efficiency of utilization of primary radicals, f, near unity. Even where the
kinetics indicate a decrease in f with dilution, the decreases have been invariably small. For undiluted
monomers, efficiencies near unity are not impossible.
Example 7.1: The following are data for the polymerization of styrene in benzene at 60°C with benzoyl
peroxide as the initiator.
[M] = 3.34 × 103 mol/m3
[I] = 4.0 mol/m3
kp2/kt = 0.95 × 10–6 m3/mol-s
If the spontaneous decomposition rate of benzoyl peroxide is 3.2 × 10–6 m3/mol-s–1, calculate the initial
rate of polymerization.
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POLYMER SCIENCE AND TECHNOLOGY
Figure 7.2 Relationship between initiator concentration and polymerization rate for different initiators: A, bis(pchlorobenzoyl)peroxide; B, benzoyl peroxide; C, acetyl peroxide in dimethylphthalate; D, lauroyl peroxide; E,
myristoyl peroxide; F, caprylyl peroxide; G, bis(2,4 dichlorobenzoyl) peroxide. [A.I. Lowell and J.R. Price, J. Poly.
Sci., 43, 1 (1996). With permission.]
Solution: For the initial rate of polymerization, Equation 7.21 is valid. Assuming the initiator efficiency
f = 1, then
R p = k p (k d k t )
12
(
[I] 1 2 [M]
)
R p2 = k d k p2 k t [I] [M]
2
m3
m3
mol
3 mol
−6
−6
4.0 3 3.34 × 10
0.95 × 10
3.2 × 10
⋅
⋅
mol
s
mol
s
m
m3
2
R p = 11.65 × 10 −3 mol m 3 − s
R p = 0.012 mol m 3 − s
Example 7.2: The data for the bulk polymerization of styrene at 60°C with benzoyl peroxide as initiator
are
[M] = 8.35 × 103 mol/m3
[I] = 4.0 mol/m3
kp2/kd = 1.2 × 10–6 m3/mol-s
If the initial rate of polymerization of styrene is 0.026 mol/m3-s and the spontaneous decomposition of
benzoyl peroxide in styrene is 2.8 × 10–6 s–1, what is the efficiency of the initiator?
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CHAIN-REACTION (ADDITION) POLYMERIZATION
Solution:
fk
Rp = kp d
kt
12
[I] 1 2 [M]
k 2
2
R p2 = fk d p [I] [M]
kt
fk d =
(0.026)2
2
1.26 × 4 × (8.35)
= 1.92 × 10 −6
f=
1.92 × 10 −6
= 0.7
2.8 × 10 −6
V. AUTOACCELERATION (TROMMSDORFF EFFECT)
Since the initiator concentration remains fairly unchanged in the course of vinyl polymerization, if the
initiator efficiency is independent of monomer concentration, first-order kinetics with respect to the
monomer is expected. This is indeed observed over a wide extent of reaction for the polymerization of
styrene in toluene solution with benzoyl peroxide as initiator (Figure 7.3). The polymerization of certain
monomers, either undiluted or in concentrated solution, shows a marked deviation from such first-order
kinetics. At a certain stage in the polymerization process, there is a considerable increase in both the
reaction rate and the molecular weight. This observation is referred to as autoacceleration or gel effect
and is illustrated in Figure 7.4 for polymerization of methyl methacrylate at various concentrations of
the monomer in benzene.
Observe that for monomer concentrations of up to 40%, plots show that first-order kinetics is followed.
However, at higher initial monomer concentrations, a sharp increase in rate is observed at an advanced
stage of polymerization. At the same time, high-molecular-weight polymers are produced. Autoacceleration is particularly pronounced with methyl methacrylate, methyl acrylate, and acrylic acid. It occurs
independent of an initiator and is observed even under isothermal conditions. In fact, where there is no
effective dissipation of heat, autoacceleration results in a large increase in temperature.
Figure 7.3 Polymerization of 40 percent styrene in toluene at 50°C in the presence of the amounts of benzoyl
peroxide shown. (G.V. Schulz and E. Husemann, Z. Physik Chem., B39, 246, 1938. With permission.)
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POLYMER SCIENCE AND TECHNOLOGY
Figure 7.4 The course of polymerization of methyl methacrylate at 50°C in the presence of benzoyl peroxide
at different concentrations of monomer in benzene. (G.V. Schulz and G. Harboth, Macromol. Chem., 1, 106, 1947.)
To understand the physical phenomena taking place during autoacceleration, let us look more carefully
at the rate equation:
fk
Rp ∝ kp d
kt
12
Since autoacceleration is not a function of the initiator, an increase in the factor fkd does not provide
an explanation for the observation. Consequently the phenomenon of autoacceleration could be due to
an increase in the propagation rate constant kp and/or a decrease in the termination rate constant kt.
Trommsdorff attributed autoacceleration to a decrease in the termination rate. As the concentration of
polymer molecules generated builds up, the viscosity of the medium increases. This reduces the overall
diffusion rate of growing radical-bearing polymer molecules even though the intrinsic reactivity of the
radical remains unaffected. As a result, the bimolecular annihilation of radical reactivity becomes
diffusion-controlled. This reduction in termination rate, in turn, increases the concentration of active
radicals, and the rate of local consumption of monomers increases proportionately. The overall rate of
propagation, however, remains relatively unaffected since the diffusion of monomers is hardly affected
by the high medium viscosity. This reduction in termination rate with little or no change in the propagation
rate leads to a rapid rise in the molecular weight.
A further reference to Figure 7.4 shows that autoacceleration does not result in complete polymerization of monomers. Also, the higher the dilution, the higher the extent of conversion. Pure poly(methyl
methacrylate) has a glass transition temperature of about 90°C. It therefore follows that when the
percentage of polymers formed reaches a certain high value, the reaction mixture is transformed into a
glass at temperatures below 90°C. Propagation becomes monomer diffusion-controlled, and there is a
virtual cessation of polymerization. There is ample experimental evidence to support the above explanation for the phenomenon of autoacceleration.
VI.
KINETIC CHAIN LENGTH
The kinetic chain length, ν, is defined as the average number of monomers consumed by each primary
radical. Obviously from this definition, the magnitude of the kinetic chain length will depend on the
rate of the propagation relative to the termination rate, i.e.,
ν=
Rp
Rt
(7.22)
Since at steady state the rate of initiation equals the rate of termination, Equation 7.22 can also be written
as
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CHAIN-REACTION (ADDITION) POLYMERIZATION
ν=
Rp
Rt
=
Rp
Ri
(7.23)
It follows on substitution for Rp and Rt from Equations 7.15 and 7.19, respectively, that
k [M]
ν= p
2 k t [M ⋅]
(7.24)
Substituting for [M·] from Equation 7.15 yields:
k 2 [M] 2
ν= p
2k t R p
(7.25)
Equations 7.24 and 7.25 are quite general and do not depend on the nature of initiation. Notice the
inverse relation between the kinetic chain length and the radical concentration. For reactions initiated
by the decomposition of an initiator,
dk
[M ⋅] = d
kt
12
[I] 1 2
Thus
ν=
kp
2 (fk d k t )
12
[M]
[I] 1 2
(7.26)
The kinetic chain length should be related to the number-average degree of polymerization. The degree
of complexity of this relation will depend on the existence or otherwise of side (chain-transfer) reactions.
We now consider first the case where there are no chain-transfer reactions. By definition
Xn =
Mo − Mt
Pt
(7.27)
where Mo = initial number of molecules of monomer present
Mt = number of monomer molecules at time t
Pt = number of polymer molecules at time t
On differentiating with respect to time, we obtain:
P( t )
d X n (t )
d P( t )
d M(t )
+ X n (t )
=−
dt
dt
dt
(7.28)
For a short interval, the instantaneous number-average degree of polymerization Xni is constant, hence
the above expression becomes
X ni = −
d M(t ) dt
d P(t ) dt
(7.29)
Recall that the rate of consumption of monomers by an active center (disappearance of monomers) is,
by definition, the rate of propagation, Rp . Now, in termination by combination, two growing chains
undergo mutual annihilation to produce a single inactive polymer molecule, whereas for termination by
disproportionation, a biomolecular annihilation of active polymer chains results in two polymers.
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POLYMER SCIENCE AND TECHNOLOGY
Consequently, dP(t)/dt = H Rt for termination by combination, while dP(t)/dt = Rt for termination by
disproportionation. From these arguments, and since at steady state Rt = Ri ,
X ni = 2 R p R t = 2 R p R i
for termination by combination
(7.30)
while
X ni = R p R t = R p R i
for termination by disproportionation
(7.31)
But kinetic chain length is given by Equation 7.23. Hence
X ni = 2 ν
for termination by combination
(7.32)
and
X ni = ν
for termination by disproportionation
(7.33)
Although experimental verifications are rare, the above expressions are valid for many systems, unless
of course chain-transfer reactions occur. We consider the case of the occurrence of chain transfer in the
next section.
Example 7.3: For pure styrene polymerized at 60°C, the value of the ratio kp2/kt is 0.0012 1/mol-s. The
corresponding value for pure vinyl acetate polymerized at 60°C is 0.125 l/mol-s.
a. Estimate their relative kinetic length chain lengths.
b. Calculate the kinetic chain length for polystyrene if the rate of polymerization is 10–4 mol/l-s and
monomer concentration is 8.35 mol/l.
Solution:
(
a. ν = k p 2 k t
)
2
[M] 2
Rp
Assume the rates of polymerization and monomer concentrations are the same for styrene(s),
and vinyl acetate (VA).
(
νVA νs = k p2 k t
) (k
VA
2
p
kt
)
s
= 0.125 0.0012 = 104
b. νs = (0.0012 2) (8.35) 10 −4 = 400
2
VII.
CHAIN-TRANSFER REACTIONS
Very frequently, a discrepancy exists between the value of the number-average degree of polymerization,
and the predicted value of the kinetic chain length ν. The number of polymer molecules is generally
found to be more than would be expected to be produced by the primary radicals (i.e., Xn is always less
than either 2 ν or ν). Obviously, other components of the reaction medium initiate chain growth. The
existence of such polymer-producing extraneous reactions is attributable to chain-transfer reactions that
consist essentially of the following sequence of steps:
M x⋅ + TL → M x L + T ⋅
T ⋅ + M → TM⋅, etc.
(7.34)
where TL = transfer agent, which may be a solvent, monomer, initiator, or polymer
L = a labile group such as hydrogen or chlorine atoms, etc.
Transfer to a polymer does not alter the number of polymer molecules produced, therefore we neglect
this type of transfer reaction in the subsequent discussion.
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CHAIN-REACTION (ADDITION) POLYMERIZATION
In the presence of transfer reactions, the expression of the kinetic chain length has to be modified to
include all possible sources of polymer molecules. Thus
ν=
Rp
Rate of All Reactions Leading to Polymer Molecule Production
(7.35)
From our discussion in the preceding section, termination mechanisms for generating polymer chains
and assuming all transfer species are as reactive as the original (i.e., no inhibition), the expression for
ν becomes:
ν=
Rp
fk d [I] + k tr ,M [M] [M ⋅] + k tr ,s [S] [M ⋅] = k tr ,I [I] [M ⋅]
(7.36)
where ktr,M ktr,S and ktr,I are velocity constants for chain transfer to monomer M, solvent S, and initiator I,
respectively. To estimate the tendency for transfer quantitatively, we define transfer constants:
CM =
k
k
k tr ,M
; CS = tr ,S ; C I = tr ,I
kp
kp
kp
(7.37)
Since Xn = ν and substituting for [I] and [M·] Equation 7.36 becomes:
2
k Rp
1
[S] + C k t R p
C
C
+
+
= t2
2
M
s
X n k p [M]
[M] I k p2 fk d [M] 3
(7.38)
2
1
[S] + k t R p + C k t R p
= C M + Cs
2
3
I 2
2
Xn
[M] k p [M]
k p fk d [M]
(7.39)
The complexity of this equation can be greatly reduced if, for example, polymerization of undiluted
monomer is done (absence of a solvent) or conditions are chosen so as to minimize or even eliminate
transfer to the initiator. In some cases substances exist whose transfer constants are equal to or exceed
unity. These substances are useful in controlling the molecular weight of the polymer. With the addition
of small quantities of such substances, known as regulators or modifiers, the molecular weight can be
depressed to a desired level. In the preparation of synthetic rubbers from butadiene or diolefins, aliphatic
mercaptans are frequently added to reduce the polymer chain length to a desired range required for
subsequent processing. The effect of chain transfer on chain length and polymer structure is summarized
in Table 7.2.
Table 7.2 Effect of Chain Transfer on Chain Length and Structure
Chain Transfer To
Small molecule resulting in
active radical
Small molecule resulting in
retardation or inhibition
Polymer (intermolecular)
Polymer (intramolecular)
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Rp
Mn
Mw
Molecular Architecture
None
Decreases
Decreases
None
Decreases
May decrease
or increase
None
None
May decrease
or increase
Increases
Increases
None
None
None
Produces long branches
Produces short branches
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POLYMER SCIENCE AND TECHNOLOGY
Example 7.4: For the polymerization of pure (undiluted) styrene with benzoyl peroxide at 60°C, the
number-average degree of polymerization, Xn, is given by the expression:
X n = 0.6 × 10 −4 + 12.0 R p + 4.2 × 10 4 R p2
where Rp = rate of polymerization (mol/l-s).
a. What is the transfer constant to the monomer, CM?
b. Calculate the transfer constant to the initiator if the monomer concentration [M] is 10 mol/l and
fkd kp2/kt = 2.29 × 10–9.
Solution: a. Comparing the above equation with Equation 7.39, it is obvious that CM = 0.6 × 10–4 and
k R p2
4
2
CI 2 t
3 = 4.2 × 10 R p
k p fk d [M]
C I = 4.2 × 10 4 [M] × 2.29 × 10 −9
3
= 0.0962
A. TRANSFER TO UNDILUTED MONOMER
For bulk polymerization (i.e., polymerization in the absence of a solvent) Equation 7.39 reduces to:
k Rp
k R p2
1
t
= C M + 2t
C
+
2
3
I 2
Xn
k p [M]
k p fk d [M]
(7.40)
This expression is quadratic in Rp, and the predicted behavior has indeed been observed in polymerization of styrene at 60° with benzoyl peroxide as the initiator. The first term on the right-hand side of
the equation represents the contribution of transfer to the monomer; it is constant and independent of
the rate of polymerization. The second term corresponds to normal termination (i.e., H ν, transfer
reactions), while the third term, which represents transfer to the initiator, increases with increasing rates
since high rates require high concentrations of initiator.
B. TRANSFER TO SOLVENT
In the presence of a solvent and with a proper choice of reaction conditions so as to minimize other
types of transfer, the general expression for transfer (i.e., Equation 7.39) becomes
1 X n = 1 X no + CS [S] [M]
(7.41)
where 1/Xno combines the terms for normal polymerization with transfer to monomer and represents the
reciprocal of the degree of polymerization in the absence of a solvent. Experimentally, in addition to
the above precautions, the ratio Rp /[M]2 is held constant while the concentration of the solvent is varied.
The transfer constants to solvents for the peroxide initiation of the polymerization of some monomers
are listed in Table 7.3.
Example 7.5: The transfer constant to the solvent for the polymerization of styrene in benzene at 100°C
is 0.184 × 10–4. How much dilution is required to halve the molecular weight given that 1/Xno = 2.5 × 10–4?
Solution: For the molecular weight to be halved, then 1/Xn becomes 2/Xno
2
1
−4
X − X = 0.184 × 10 [S] [M]
no
no
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CHAIN-REACTION (ADDITION) POLYMERIZATION
1 10 4
[S]
X 0.184 = M
[ ]
no
[S] = 13.6 [M]
Dilution factor is about 14.
Table 7.3 Transfer Constants to Solvents for the Peroxide Initiation
of Monomer Polymerization at 60°C
Transfer Constant: CS × 104
Solvent
Methyl Methacrylate
Styrene
Vinyl Acetate
0.195
0.83
2.40
1.77
0.202
4.1
0.028
87.0
3.4
0.105
25.6 (70°C)
2.4
2023.0 (70°C)
554.0 (70°C)
20.75
Acetone
Benzene
Carbon tetrachloride
Chloroform
Toluene
From Schulz, G.V. and Harborth, H., Makromol. Chem., 1, 106, 1947. With permission.
VIII.
TEMPERATURE DEPENDENCE OF DEGREE OF POLYMERIZATION
The dependence of the polymerization rate and the number-average degree of polymerization on temperature can be demonstrated if the respective relation is expressed in the form of the Arrhenius equation.
k b = A exp
d ln R p
dT
−Eb
RT
E E
E − t + d
p 2 2
=
RT 2
E E
E − t − d
d ln X n p 2 2
=
dT
RT 2
(7.42)
(7.43)
(7.44)
For radical polymerizations the activation energy of decomposition is of the order of 30 kcal/mol while
(Ep – Et /2) is about 4 to 7 kcal/mol. Thus the temperature coefficients are, respectively,
dR p dT > 0
(7.45)
dX n dT < 0
(7.46)
These results predict that the rate of polymerization increases with increasing temperature while the
molecular weight decreases. The same conclusion can be drawn for thermal polymerizations since (Ep –
E t /2) – E i /2 is a negative quantity.
Example 7.6: The energies of activation for the polymerization of styrene with di-tertiary-butyl peroxide
as initiator are
Ed = 33.5 kcal/mol
Ep = 7.0 kcal/mol
Et = 3.0 kcal/mol
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POLYMER SCIENCE AND TECHNOLOGY
Calculate the relative (a) rates of propagation and (b) degree of polymerization (Xη) if the polymerization
temperature is changed from 50°C to 60°C.
Solution:
a.
d ln R p
dT
=
where E = E p −
∫
Rp T
Rp T
o
E
RT 2
Et Ed
+
.
2
2
d ln R p =
∫
R pT
E
R
ln
R pTo
=
T
E
dT
2
To RT
1
1
−
To T
To = 50°C = 323 K ; T = 333 K
E = 7−
3 33.5
+
= 22.25 kcal mol
2
2
E 22.25 × 10 3 cal mol
=
1.99 cal mol − K
R
1
1
−
= 0.0001 K −1
T To
ln
R p 60
R p 50
= 1.12
R p 60 R p 50 = l1.12 = 3.06
ln X n 60 X n 50 =
E
RT
1 1
Et Ed
−
− ; E − Ep −
2
2
To T
= −11.25 kcal mol
11.25 × 103 cal mol
−1
E RT = −
0.001 K
1.99 cal mol − K
(
)
= −0.57
X η60
X η50
IX.
= l −0.57 = 0.57
IONIC AND COORDINATION CHAIN POLYMERIZATION
A. NONRADICAL CHAIN POLYMERIZATION
In addition to the radical chain polymerization mechanisms discussed above, chain-reaction polymerization can also occur through other mechanisms. These include cationic polymerization in which the chain
carriers are carbonium ions; anionic polymerization where the carriers are carbanions; and coordination
polymerization, which is thought to involve the formation of a coordination compound between the
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CHAIN-REACTION (ADDITION) POLYMERIZATION
catalyst, monomer, and growing chain. The polymerization mechanisms of these systems are complex
and not as clearly understood as the mechanism of radical polymerization. This is because the reactions
are generally heterogeneous, involving usually solid inorganic catalysts and organic monomers. In
addition, ionic polymerizations are characterized by extremely high reaction rates. High-molecularweight polymers are generated so fast that it is frequently neither possible to establish nor maintain
uniform reaction conditions, thus making it difficult to obtain kinetic data or reproducible results. Two
essential differences between free-radical and ionic polymerizations are apparent. First, in ionic polymerization, initiation involves the formation of an ion pair through the transfer of an ion or electron to
or from the monomer. This contrasts with the generation and addition of a radical to the monomer in
free-radical initiation reactions. Second, termination in ionic polymerization involves the unimolecular
reaction of a chain with its counterion or a transfer reaction with the remnant species unable to undergo
propagation. In contrast to radical chain polymerization, this termination process in ionic polymerization
is strictly unimolecular — bimolecular annihilation of growth activity between two growing chains does
not occur. The types of chain polymerization suitable for common monomers are shown in Table 7.1.
While many monomers can polymerize by more than one mechanism, it is evident that the polymerization mechanism best suited for each monomer is related to the polarity of the monomer and the Lewis
acid–base strength of the ion formed. Monomers in which electron-donating groups are attached to the
carbon atoms with the double bond (e.g., isobutylene) are capable of forming stable carbonium ions (i.e.,
they behave as Lewis bases). Such monomers are readily converted to polymers by cationic catalysts (Lewis
acids). On the other hand, monomers with electron-withdrawing substituent (e.g., acrylonitrile) form stable
carbanions and polymerize with anionic catalysts. Free-radical polymerization falls between these structural
requirements, being favored by conjugation in the monomer and moderate electron withdrawal from the
double bond. The structural requirements for coordination polymerization are less clearly delineated, and
many monomers undergo coordination polymerization as well as ionic and radical polymerizations.
B. CATIONIC POLYMERIZATION
Typical catalysts that are effective for cationic polymerization include AlCl3, AlBr3, BF3, TiCl4, SnCl4,
and sometimes H2SO4. With the exception of H2SO4, these compounds are all Lewis acids with strong
electron-acceptor capability. To be effective, these catalysts generally require the presence of a Lewis
base such as water, alcohol, or acetic acid as a cocatalyst. As indicated in Table 7.1, monomers that
polymerize readily with these catalysts include isobutylene, styrene, α-methylstyrene and vinyl alkyl
ethers. All of these monomers have electron-donating substituents, which should enhance the electronsharing ability of the double bonds in these monomers with electrophilic reagents.
Cationic polymerizations proceed at high rates at low temperatures. For example, the polymerization
at –100°C of isobutylene with BF3 or AlCl3 as catalysts yields, within a few seconds, a polymer with
molecular weight as high as 106. Both the rate of polymerization and the molecular weight of the polymer
decrease with increasing temperature. The molecular weights of polyisobutylene obtained at room
temperature and above are, however, lower than those obtained through radical polymerization.
1. Mechanism
Based on available experimental evidence, the most likely mechanism for cationic polymerization
involves carbonium ion chain carrier. For example, the polymerization of isobutylene with BF3 as the
catalyst can be represented thus:
First, the catalyst and cocatalyst (e.g., water) form a complex:
BF3 + H2O
H+(BF3OH)-
(7.47)
The complex then donates a proton to an isobutylene molecule to form a carbonium ion:
CH3
H+(BF3OH)- + CH2
C
CH3
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CH3
CH3
C+ + (BF3OH)CH3
(7.48)
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POLYMER SCIENCE AND TECHNOLOGY
The carbonium ion reacts with a monomer molecule in the propagation step.
CH3
CH3
CH3
M
C+ (BF3OH)-
(CH3)3C
CH2
CH3
C+ (BFOH)-
(7.49)
CH3
In general, the propagation reaction can be written as
CH3
CH3
C
CH3
CH2
CH3
+
CH3
M
+ (BF3OH)-
C
CH3
(CH3)3
C
CH2
C
CH3
n
+
+ (BF3OH)-
(7.50)
n+1
Since cationic polymerization is generally carried out in hydrocarbon solvents that have low dielectric
constants, separation of the ions would require a large amount of energy. Consequently, the anion and
cation remain in close proximity as an ion pair. It is therefore to be expected that the growth rate and
subsequent reactions (e.g., termination and chain transfer) are affected by the nature of the ion pair.
Termination occurs either by rearrangement of the ion pair to yield a polymer molecule with an
unsaturated terminal unit and the original complex or through transfer to a monomer.
CH3
CH3
C (BF3OH)+
CH2
CH2
CH3
CH2
C
(7.51)
CH3
CH3
+
C + H+ (BF3OH)-
(BF3OH)-
M
CH2
CH3
CH2
CH3
C + CH3
C+ + (BF3OH)-
CH3
CH3
(7.52)
Unlike in free-radical polymerization, the catalyst is not attached to the resulting polymer molecule, and
in principle many polymer molecules can be produced by each catalyst molecule.
2. Kinetics
For the purpose of establishing the kinetics of generalized cationic polymerization, let A represent the
catalyst and RH the cocatalyst, M the monomer, and the catalyst–cocatalyst complex H+ AR–. Then the
individual reaction steps can be represented as follows:
A + RH 1 H + AR −
K
H + AR − + M i → HM + AR −
k
kp
HM n+ AR − + M → HM n++1 AR −
(7.53)
HM n+ AR − t → M n + H + AR −
k
tr
HM n+ AR − + M
→ M n + HM + AR −
k
The rate of initiation Ri is given by
[
]
R i = k i H + AR − [M] = k i K [A] [RH] [M]
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(7.54)
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CHAIN-REACTION (ADDITION) POLYMERIZATION
As usual, the square brackets denote concentration. If the complex H+ AR– is readily converted in the
second step of Equation 7.48 (i.e., if the complex formation, step 1 Equation 7.48) is the rate-limiting
step, then the rate of initiation is independent of the monomer concentration. Since AR– remains in the
close vicinity of the growing center, the termination step is first order
[ ]
Rt = Kt M+
(7.55)
where [M+] is the concentration of all the chain carriers [HM+n AR–]. The retention of the terminating
agent AR– in the vicinity of the chain carrier is responsible for the primary difference between the
kinetics of cationic polymerization and that of free-radical polymerization. Assuming that steady state
holds, then Ri = Rt and
[M ] = Kkk
+
i
[A] [RH] [M]
(7.56)
t
The overall rate of polymerization, Rp is given by
[ ]
R p = k p M + [M] = K
ki kp
kt
[A] [RH] [M] 2
(7.57)
The number-average degree of polymerization, assuming predominance of termination over chain transfer, is
Xn =
Rp
Ri
=
[ ]
[M ]
k p M + [M]
kt
+
=
kp
kt
[M]
(7.58)
If, on the other hand, chain transfer dominates, then
Xn =
Rp
R tr
=
[ ] =k
k [M ] [M] k
k p M + [M]
p
+
t
(7.59)
tr
In this case, the average degree of polymerization is independent of both the concentration of the monomer
and the concentration of the catalyst. Available kinetic data tend to support the above mechanism.
Example 7.7: Explain why in the cationic polymerization of isobutylene, liquid ethylene or propylene
at their boiling points are normally added to the reaction medium as a diluent. How will an increase in
the dielectric constant of the reaction medium affect the rate and degree of polymerization?
Solution: Both the rate of polymerization and the molecular weight decrease with increasing temperature
in cationic polymerization. These liquids help to prevent excessive temperature increases because part
of the heat of polymerization is dissipated through the heat of evaporation of the liquids. In other words,
the liquids act essentially as internal refrigerants.
Using the general relation between the rate of reaction and activation energy (k = Ae–E/RT), we note that
a decrease in the activation energy, E, increases the rate of reaction while an increase in E has the
opposite effect. An increase in dielectric constant increases the rate of initiation, ki, by reducing the
energy required for charge separation. On the other hand, an increase in the dielectric constant decreases
kt by increasing the energy required for the rearrangement and combination of the ion pair. Since both
Rp and Xn are directly proportional to ki/kt, an increase in dielectric constant increases both quantities.
C. ANIONIC POLYMERIZATION
Monomers with electronegative substituents polymerize readily in the presence of active centers bearing
whole or partial negative charges. For example, a high-molecular-weight polymer is formed when
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200
POLYMER SCIENCE AND TECHNOLOGY
methacrylonitrile is added to a solution of sodium in liquid ammonia at –75°C. Typical electronwithdrawing substituents that permit the anionic polymerization of a monomer include –CN, –COOR,
–C6H5, and –CH› CH2. The electronegative group pulls electrons from the double bond and consequently
renders the monomer susceptible to attack by an electron donor. Catalysts for anionic polymerization
include Grignard reagents, organosodium compounds, alkali metal amides, alkoxide, and hydroxides.
1. Mechanism
Propagation in anionic polymerization proceeds according to the following reactions:
X
CH2
X
C - M+
+
CH2
X
C
Y
CH2
C
Y
X
CH2
Y
C - M+
(7.60)
Y
Here, M+ represents a counterion that accompanies the growing chain. In most cases, M+ is an alkali
metal ion, whereas X and Y are either electron-withdrawing groups or unsaturated groups capable of
resonance stabilization of the negative charge.
Initiation may occur in two ways: a direct attack of a base on the monomer to form a carbanion
(Equation 7.61) or by transfer of an electron from a donor molecule to the monomer to form an anion
radical (Equation 7.62).
X
M + B-
+
CH2
X
C
B
CH2
Y
+
CH2
(7.61)
Y
X
MO
C - M+
C
Y
X
• CH
C - M+
2
(7.62)
Y
anion radical
M+B– may be a metal amide, alkoxide, alkyl, aryl, and hydroxide depending on the nature of the monomer.
The effectiveness of the catalyst in the initiation process depends on its basicity and the acidity of the
monomer. For example, in the anionic polymerization of styrene, the ability to initiate reaction decreases
in the order
–CH2––NH –2 > NH –2 @ OH–. Indeed, OH– will not initiate anionic polymerization of
styrene. Where the anion is polyvalent, such as tris(sodium ethoxy) amine N(CH2CH2O– Na+) 3 , an
equivalent number of growing chains (in this case, three) can be initiated simultaneously.
The donor molecular in Equation 7.58 represents, in general, an alkali metal. In this case, transfer
results in the formation of a positively charged alkali metal counterion and an anion radical. Pairs of
anion radicals combine to form a dianion.
• CH
2
X
X
C - M+
C - M+
Y
+
• CH
2
Y
X
M+ C Y
X
CH2
CH2
C - M+
(7.63)
Y
In carefully controlled systems (pure reactants and inert solvents), anionic polymerizations do not
exhibit termination reactions. As we shall see shortly, such systems are referred to as living polymers;
however, because of the reactivity of carbanions with oxygen, carbon dioxide, and protonic compounds,
termination occurs according to Equation 7.64.
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201
CHAIN-REACTION (ADDITION) POLYMERIZATION
C- + O 2
COO-
C- + CO2
CC
(a)
(7.64)
O
(b)
OC- + H2O
CH + OH-
(c)
C- + RH
CH + R-
(d)
O
O
C- + RC
CH + RC
(e)
O-
OH
The terminal groups in Equations 7.64a and 7.64b cannot propagate and, consequently, effectively
terminate polymer growth. Equations 7.64c–7.64e involve proton transfer from the solvent to the growing
chain resulting in a dead polymer. This is exemplified by the sodium-catalyzed polymerization of
butadiene in toluene:
3C 6 H 5
∧∧− CH − CH› CH − CH − Na + CH
→ ∧∧− CH 2 − CH› CH − CH 3 + C 6 H 5CH 2− Na +
2
2
( toluene )
Other possible termination reactions include (1) the transfer of a hydride ion leaving a residual terminal
unsaturation (this, however, is a high-energy process and therefore unlikely); (2) isomerization of the
carbanion to give an inactive anion; and (3) irreversible reaction of the carbanion with the solvent or
monomer. In general, termination by transfer to the solvent predominates in anionic polymerization.
2. Kinetics
Available kinetic data for the polymerization of styrene by potassium amide in liquid ammonia support
the following steps in the mechanism of anionic polymerization.
KNH 2 1 K + + NH 2−
K
ki
NH 2− + M
→ NH 2 M −
(7.65)
NH 2 M n− + M p → NH 2 M n−+1
k
tr
NH 2 M n− + NH 3
→ NH 2 M n H + NH 2−
k
Considering the relatively high dielectric constant of the liquid ammonia medium, the counterion K+
can be neglected. Assuming steady-state kinetics:
[
]
R i = k i NH 2− [M]
[
(7.66)
][
R t = k tr NH 2 − M n− NH 3
]
(7.67)
Thus from Equations 7.61 and 7.62
[
Copyright 2000 by CRC Press LLC
]
NH 2 − M n− =
[
]
−
k i NH 2 [M]
k tr
NH3
[
]
2
(7.68)
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POLYMER SCIENCE AND TECHNOLOGY
The rate of polymerization becomes
[
]
R p = k p NH 2 − M n− [M]
= kp
[
]
−
k i NH 2 [M]
k tr
NH3
[
(7.69)
2
]
Given the predominance of transfer reactions, the degree of polymerization Xn is given by
Xn =
Rp
Rt
=
kp
[
[M]
k tr NH3
]
(7.70)
Example 7.8: Polymerization of styrene in liquid ammonia gave high yield of low-molecular-weight
polystyrene, whereas methacrylonitrile gave high conversion of high-molecular-weight polymethacrylonitrile. Explain.
Solution: Since the reaction is carried out in liquid ammonia, proton transfer from the solvent (ammonia)
is the possible termination reaction. The relevant carbanions are
H
CH2
CH3
C
and
CH2
C
(Str. 4)
CN
Since the –CN group is more electronegative than the phenyl group, the styrene carbanion will be more
basic than the methacrylonitrile carbanion and as such more susceptible to proton transfer from ammonia.
Transfer reactions generally lead to low-molecular-weight polymers.
D. LIVING POLYMERS
The absence of the termination step in anionic polymerizations with carefully purified reactants in inert
reactions media results, as indicated above, in living polymers. In such systems, the growing species
remain dormant in the absence of monomers but resume their growth activity with a fresh monomer
supply. With adequate mixing, the monomer supplied to the system is distributed among the growing
centers (living polymers). As a result, the number-average degree of polymerization is simply the ratio
of the number of moles of monomer added to the total number of living polymers. That is,
Xn =
[monomer]
[catalyst]
Unlike other chain reactions where the growing chains undergo spontaneous termination, the reactive
end groups in living polymers may be annihilated (“killed”) by a choice of suitable reactants by the
experimenter at a desired stage of the polymerization process. Thus living polymers offer fascinating
potentials for the development of novel polymer systems. We discuss a few examples.
Spontaneous termination is governed by the laws of probability. Therefore, in polymerization involving spontaneous termination, the resulting polymers have a distribution of molecular weights since the
lifetimes of the growing centers vary. But with living polymers where termination is lacking, a polymer
with a nearly uniform molecular weight (monodisperse) can be obtained if the system is devoid of
impurities and is well mixed.
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CHAIN-REACTION (ADDITION) POLYMERIZATION
203
The ability of living polymers to resume growth with the addition of fresh monomer provides an
excellent opportunity for the preparation of block copolymers. For example, if a living polymer with
one active end from monomer A can initiate the polymerization of monomer B, then an A–AB–B type
copolymer can be obtained (e.g., styrene–isoprene copolymer). If, however, both ends of polymer A are
active, a copolymer of the type B–BA–AB–B results. Examples are the thermoplastic rubbers polystyrene-polyisoprene-polystyrene and poly(ethylene oxide)-polystyrene-poly(ethylene oxide). In principle,
for fixed amounts of two monomers that are capable of mutual formation of living polymers, a series
of polymers with constant composition and molecular weight but of desired structural pattern can be
produced by varying the fraction and order of addition of each monomer.
The potential versatility of living polymers in chemical synthesis is further demonstrated by the
possibility of formation of polymers with complex shapes by employing polyfunctional initiators or
terminating monodisperse living polymers with polyfunctional linking agents. For example, star-shaped
poly(ethylene oxide) can be prepared with the trifunctional initiator trisodium salt of triethanol amine,
N(CH2CH2O– Na+)3. Another possible area of utilizing living polymers is in the introduction of specific
end groups by terminating the living polymer with an appropriate agent. For example, termination of
living polystyrene with CO2 introduces terminal carboxylic groups, while reaction with ethylene oxide
introduces hydroxy end groups. The utilization of these two approaches (synthesis of block copolymers
or functional-ended polymers) provides the synthetic polymer chemist with a powerful tool for producing
polymers with fascinating architectural features and properties (Table 7.4).
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POLYMER SCIENCE AND TECHNOLOGY
E. COORDINATION POLYMERIZATION
A major development in polymer chemistry was the development in 1953 of new catalysts leading to
the formation of polymer with exceptional structural regularity. The first catalysts were described by
Ziegler7 for the low-pressure polymerization of ethylene. These were modified by Natta8 and his associates and used for the highly stereospecific polymerization of α-olefins, diolefins, and other monomers.
As indicated in Chapter 1, Ziegler and Natta were awarded the Nobel prize in chemistry in 1963 for
their work in this area. These catalysts are usually referred to as Ziegler–Natta catalysts, and since
polymerization processes utilizing these catalysts result in stereoregular structures, they are sometimes
called stereospecific or stereoregular polymerization. However, the term coordination polymerization is
used here to reflect the mechanism which, as we shall see presently, is believed to govern the reaction
involving these catalysts.
1. Mechanisms
The metals that are more frequently found as components of Ziegler–Natta catalysts are some light
elements of groups I–III of the periodic table (e.g., Li, Be, Mg, Al), present as organometallic compounds
and halides, or other derivatives of transition metals of groups IV–VIII (e.g., Ti, V, Cr, Mo, Rh, Rn, Co,
and Ni). A typical example is the product(s) of the reaction between triethylaluminum and titanium
tetrachloride. The composition of the product is not well defined but is believed to be either an alkylated
metal halide (monometallic I) or a bimetallic complex involving a bridge between the two metals (II).
C2 H5 Ti Cl3 + (C2 H5)2 AlCl
(I)
Ti Cl4 + (C2 H5)3 Al
(Str. 5)
Cl
Cl
Ti
C2H5
Al
Cl
CH2
(II)
CH3
C2H5
While coordination polymerization may be anionic or cationic, relatively fewer examples of cationic
coordination polymerization leading to the formation of stereoregular polymers currently exist. These
are limited almost exclusively to the polymerization of monomers containing heteroatoms with lone
electron pairs such as oxygen or nitrogen. In any case, the growth reaction in coordination polymerization
is considered to be controlled by the counterion of the catalysts, which first coordinates and orients the
incoming monomer and then inserts the polarized double bond of the monomer in the polarized bond
between the counterion and the end of the growing chain. Coordination involves the overlap of the
electrons of the monomer with a vacant sp orbital in the case of groups I–III metals or a vacant d orbital
in the case of transition metals. The proposed propagation mechanisms for both the monometallic and
bimetallic catalyst are shown below.
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205
CHAIN-REACTION (ADDITION) POLYMERIZATION
Cl
Cl Cl
C
CH
Cl
(Str. 6)
CHR
Ti
Cl
5
2
CH
Cl
CH2
2
H
5
CHR
2
C
Ti
2
CH
Cl
2
CH
CHR
2
H
5
2
2
CH2
CH2
Cl
2
2
2
CH
CH
R
CH
C
H
Cl
Ti
5
Cl
2
CH
CH
C
H
Monomettalic catalyst
Ti
Cl Cl
(Str. 7)
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POLYMER SCIENCE AND TECHNOLOGY
The dominant feature of the coordination polymerization mechanism is the presence of forces that orient
and insert each incoming monomer into the growing polymer chain according to a particular steric
configuration. The surface of the crystalline salts of the transition metals that constitute part of the
catalyst system is thought to play a vital role in this function. With Ziegler–Natta catalysts, ethylene is
polymerized to a highly linear chain compared with the branched products from the high-pressure process
(radical polymerization). By a convenient choice of catalyst system, isotactic and syndiotactic polypropylene can be obtained. Higher α-olefins yield isotactic polymers with heterogeneous catalyst systems.
The versatility and selectivity of Ziegler–Natta catalysts are demonstrated even more sharply in the
polymerization of conjugated dienes. By a suitable choice of catalyst system and reaction conditions,
conjugated dienes like butadiene and isoprene can be made to polymerize into any of their isomers
almost exclusively: trans-1,4; cis-1,4; or isotactic or syndiotactic 1,2.
Example 7.9: Among the elements of the first group of the periodic table, only organometallic compounds of lithium show a considerable tendency to give stereospecific catalysts. Also, this tendency
decreases with increasing atomic radius of the elements of this group. Explain this observation.
Solution: Two conditions that are essential for anionic coordination polymerization are (1) the monomer
must form a complex with the metal atom of the catalyst before its insertion in the metal–carbon bond,
and (2) the bond between the metal of the catalyst and the carbon atom of the polymer chain must be
at least partially polarized. Now, the electropositive nature of the metals of the first group of the periodic
table increases with increasing atomic radius. This means that among these elements lithium has the
highest electron-withdrawing power (generates the highest electric field) and consequently has the highest
ability to coordinate with the double bond of the monomer (condition 1). The ionization of the bond
between the metal (M) and the polymer (P) may be represented as follows:
M
P
covalent
(I)
Mδ+
polarized
(II)
P δ-
M+ +
free ions
(III)
P-
(Str. 8)
The degree of ionization of this bond depends partly but significantly on the electropositivity of the
metal. Only the polarized metal–polymer bond of type II leads to the formation of sterically regular
polymers. When a metal is very electropositive, it can be assumed that the growing polymer anion is
essentially a free ion (III) and as such the positive counterion has negligible influence on the coordination,
orientation, and insertion of the incoming monomer into the polymer chain and hence on the structural
regularity of the polymer. Consequently, the tendency for stereospecificity decreases with increasing
electropositivity (i.e., increasing atomic radius).
Example 7.10: Explain the observed variations in the stereoregularity of polymers when the metal of
the catalyst or the reaction medium is varied.
Solution: The growth reaction in coordination polymerization first involves coordination of the monomer to the counterion of the catalyst and the insertion of the polarized double bond of the monomer in
the polarized bond between the counterion and the end group of the growing chain. The nature of the
coordinating metal in the catalyst determines the particular orientation imposed on the absorbed monomer. Consequently, stereoregularity of polymer may vary with different metals. The ability of the
counterion to determine the orientation imposed on the incoming monomer is influenced by the extent
of polarization of the bond between the counterion and the end group of the growing chain. The extent
of polarization of this bond is itself a function of the solvating power of the reaction medium. Therefore,
variation of the reaction medium causes a variation in polymer stereoregularity.
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207
CHAIN-REACTION (ADDITION) POLYMERIZATION
X.
PROBLEMS
7.1. The following data were obtained by Arnett12 for the polymerization of methyl methacrylate in benzene
at 77°C with azo-bis-isobutyronitrile initiator. Assuming that the initiator efficiency is independent of
monomer concentration, are the data consistent with the model for the rate of polymerization by freeradical mechanism?
[M]
(kmol/m3)
[I]
(mol/m3)
–d[M]/dt
(mol/m3 · sec)
9.04
8.63
7.19
6.13
4.96
4.75
4.22
4.17
3.26
2.07
0.235
0.206
0.255
0.228
0.313
0.192
0.230
0.581
0.245
0.211
0.193
0.170
0.165
0.129
0.122
0.0937
0.0867
0.130
0.0715
0.415
From Arnett, L.M., J. Am. Chem. Soc.,74,
2027, 1952. With permission.
7.2. A steady-state free-radical styrene polymerization process is being controlled such that the rate of
polymerization is constant at 1.79 g of monomer/ml-min. The initiator concentration is 6.6 × 10–6 mol/ml.
a. What must be done to maintain the constant rate of polymerization?
b. If the rate constant for the first-order decomposition of the initiator, kd, is 3.25 × 10–4 min–1, what is
the rate of free radical generation per second per milliliter? What is Xn?
c. What percentage of the original initiator concentration remains after a reaction time of 3 h?
7.3. Consider the isothermal solution polymerization of styrene at 60°C in the following formulation:
100 g styrene
400 g benzene
0.5 g benzoyl peroxide
Assume that the initiator is 100% efficient and has a half-life of 44 h. At 60°C, kp = 145 l/mol-s, kt =
0.130 l/mol-s. All ingredients have unit density.
a. Derive the rate expression for this polymerization reaction.
b. Calculate the rate of propagation at 50% conversion.
c. How long will it take to reach this conversion?
7.4. For the polymerization of pure (undiluted) styrene with benzoyl peroxide at 60°C, the number-average
degree of polymerization, Xn, is given by the general expression:
R
R2
1
= 0.60 × 10 −4 + 8.4 × 10 2 p 2 + 2.4 × 10 7 p 3
Xn
[M]
[M]
For a rate of polymerization of 10–4 mol/l-s and monomer concentration of 8.35 mol/l calculate:
a. The value of the transfer constant to the monomer
b. Number-average degree of polymerization, Xn
c. Xn assuming there is no transfer (normal termination)
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208
POLYMER SCIENCE AND TECHNOLOGY
d. The kinetic chain length
e. The transfer constant to the initiator if fkdk 2p/k t = 2.29 × 10–9
f. The efficiency of initiation of polymerization of styrene in benzoyl peroxide if kd = 3.2 × 10–6 s–1
7.5. The bulk polymerization of styrene at 100°C with benzoyl peroxide as the initiator resulted in a polymer
of molecular weight 4.16 × 105. End-use tests showed that this product would be adequate provided the
variation in molecular weight did not exceed 20%. However, to ensure better temperature control of the
reactor, it was decided that the polymerization should be carried out in a solvent at a dilution factor
of 2. The following solvents are available:
Transfer Constant, CS,
for styrene at 100°C
Solvent
0.16 × 10–4
180.0 × 10–4
Cyclohexane
Carbon tetrachloride
Assuming that only transfer to the solvent is possible, show that cyclohexane is the better of the two
solvents for the end use under calculation.
7.6. The transfer constants to the solvent for the polymerization of styrene, methyl methacrylate, and vinyl
acetate in toluene at 80°C are given below:
Chain radical
Styrene
Methyl methacrylate
Vinyl acetate
CS × 104
0.31
0.52
92.00
Find the ratio of the dilution factors for the three monomers if the molecular weight of the resulting
polymers is each reduced to one-fourth that from the solvent-free polymerization. Assume that the
degree of polymerization Xn in the absence of the solvent for the monomers is the same [Xno = 5.0 × 103].
7.7. Explain why molecular weight (Xn) increases with decreasing reaction temperature in cationic polymerization. Assume that termination predominates over transfer.
7.8. The rate constant for propagation of (polystyrene)– Na+ in tetrahydrofuran for 25°C was found to be
400 l-mol–1 s–1. The rate constants for proton transfer to the anion from water and ethanol were 4000 l-mol–1
s–1 and 4 l-mol–1 s–1, respectively. Which of these impurities (water or ethanol) is more likely to inhibit
high polymer formation in sodium-catalyzed anionic polymerization of styrene in tetrahydrofuran?
7.9. If anionic coordination is carried out in ethers or amines, sterically regular structures are generally not
obtained. Explain this observation.
7.10. One mole of styrene monomer and 1.0 × 10 mol of azo-bis-isobutyronitrile initiator are dissolved in 1 l
benzene. Estimate the molecular weight of the resulting polymer if each initiator fragment starts a chain and
a. All chains start at the same time. Termination is by disproportionation (all chains are of equal length).
b. Same as in case (a) but termination is by coupling.
c. Same as in case (a) but 6.0 × 10–4 mol mercaptan are added and each mercaptan acts as a chain
transfer agent once.
7.11. The following data are obtained for the polymerization of a new monomer. Determine
a. The time for 50% conversion in run D
b. The activation energy of the polymerization
Run
Temperature
(°C)
Conversion
(%)
Time (min)
Initial Monomer
Conc. (mol/l)
Initial Initiator
Conc. (mol/l)
A
B
C
D
60
80
60
60
50
75
40
50
50
700
60
—
1.0
0.50
0.80
0.25
0.0025
0.0010
0.0010
0.0100
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CHAIN-REACTION (ADDITION) POLYMERIZATION
209
7.12. Sketch, on the same plot, molecular weight conversion curves for
a. Free-radical polymerization
b. Living polymerization
c. Condensation polymerization
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
Lenz, R.W., Organic Chemistry of High Polymers, Interscience, New York, 1967.
Lowell, A.I. and Price, J.R., J. Polym. Sci., 43, 1, 1960.
Schulz, G.V. and Husemann, E., Z. Phys. Chem. Abt. B., 39, 246, 1938.
Schulz, G.V. and Harborth, H., Macromol. Chem., 1, 106, 1947.
Young, L.J., in Polymer Handbook, 2nd ed., Brandrup, J. and Immergut, E.H., Eds., John Wiley & Sons, New York,
1975.
Webster, O.W., Science, 251, 887, 1991.
Ziegler, K., Holzkamp, E., Breil, H., and Martin, H., Agnew. Chem., 67, 541, 1955.
Natta, G., Pino, P., Corradini, P., Danusso, F., Mantica, E., Mazzanti, G., and Moraghio, G., J. Am. Chem. Soc., 77,
1708, 1955.
Flory, P.J., Principles of Polymer Chemistry, Cornell University Press, Ithaca, NY, 1953.
Odian, G., Principles of Polymerization, McGraw-Hill, New York, 1970.
Billmeyer, F.W., Jr.: Textbook of Polymer Science, 2nd ed., Interscience, New York, 1971.
Arnett, L.M., J. Am. Chem. Soc., 74, 2027, 1952.
Copyright 2000 by CRC Press LLC
Chapter 8
Copolymerization
I.
INTRODUCTION
As indicated in Chapter 1, the polymerization of organic compounds was first reported about the mid19th century. However, it was not until about 1910 that the simultaneous polymerization of two or more
monomers (or copolymerization) was investigated when it was discovered that copolymers of olefins
and dienes produced better elastomers than either polyolefins or polydienes alone. The pioneering work
of Staudinger in the 1930s and the development of synthetic rubber to meet wartime needs opened the
field of copolymerization.
Copolymers constitute the vast majority of commercially important polymers. Compositions of
copolymers may vary from only a small percentage of one component to comparable proportions of
both monomers. Such a wide variation in composition permits the production of polymer products with
vastly different properties for a variety of end uses. The minor constituent of the copolymer may, for
example, be a diene introduced into the polymer structure to provide sites for such polymerization
reaction as vulcanization; it may also be a trifunctional monomer incorporated into the polymer to ensure
cross-linking, or possibly it may be a monomer containing carboxyl groups to enhance product solubility,
dyeability, or some other desired property. Copolymerization reactions may involve two or more monomers; however, our discussion here is limited to the case of two monomers.
II.
THE COPOLYMER EQUATION
Some observations are relevant to the consideration of copolymerization kinetics are
• The number of reactions involved in copolymerization of two or more monomers increases geometrically with the number of monomers. Consequently, the propagation step in the copolymerization of
two monomers involves four reactions.
• The number of radicals to be considered equals the number of monomers. The terminal monomer unit
in a growing chain determines almost exclusively the reaction characteristics; the nature of the preceding
monomers has no significant influence on the reaction path.
• There are two radicals in the copolymerization of two monomers. Consequently, three termination steps
need to be considered.
• The composition and structure of the resulting copolymer are determined by the relative rates of the
different chain propagation reactions.
By designating the two monomers as M1 and M2 and their corresponding chain radicals as M1· and
M2·, the four propagation reactions and the associated rate equations in the copolymerization of two
monomers may be written as follows:
Reaction
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© 1997 by CRC Press LLC
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Rate equation
[ ][ ]
M1⋅ + M1 → M1⋅
k11 M1⋅ M1
M1⋅ + M 2 → M 2 ⋅
k12 M1⋅ M 2
M 2 ⋅ + M1 → M1⋅
k 21 M2 ⋅ M1
M2⋅ + M2 → M2⋅
k 22 M2 ⋅ M 2
[ ][ ]
[
][ ]
[
][ ]
(8.1)
211
212
POLYMER SCIENCE AND TECHNOLOGY
Here the first subscript in the rate constant refers to the reacting radical, while the second subscript
designates the monomer. Now, it is reasonable to assume that at steady state, the concentrations of M1·
and M2· remain constant. This implies that the rates of generation and consumption of these radicals are
equal. It follows therefore that the rate of conversion of M1· to M2· necessarily equals that of conversion
of M2· to M1·. Thus from Equation 8.1
[
][ ]
[ ][ ]
k 21 M 2 ⋅ M1 = k12 M1⋅ M 2
(8.2)
The rates of disappearance of monomers M1 and M2 are given by
[ ] = k [ M ⋅][M ] + k [ M ⋅][M ]
− d M1
11
dt
1
1
21
2
1
[ ] = k [ M ⋅][M ] + k [ M ⋅][M ]
dt
−d M2
12
1
2
22
2
1
(8.3)
(8.4)
By using Equation 8.2, one of the radicals can be eliminated. By dividing Equation 8.3 by Equation 8.4
we obtain:
[ ] = [M ] r [M ] + [M ]
d [M ] [M ] [M ] + r [M ]
d M1
1
2
2
1
2
2
1
2
(8.5)
2
where r1 and r2 are monomer reactivity ratios defined by
r1 = k11 k12
and
(8.6)
r2 = k 22 k 21
Equation 8.5 is the copolymer equation. Let F1 and F2 represent the mole fractions of monomers M1 and
M2 in the increment of polymer formed at any instant during the polymerization process, then
[ ] ([ ] [ ])
F1 = 1 − F2 = d M1 d M1 + M 2
(8.7)
Similarly, representing the mole functions of unreacted M1 and M2 in the monomer feed by f1 and f2, then
f1 = 1 − f2 =
[M ]
[ ] [ ]
1
M1 + M 2
(8.8)
Substitution of Equations 8.7 and 8.8 in Equation 8.5 yields:
F1 =
III.
r1 f12 + f1 f2
r f + 2 f1 f2 + r2 f22
2
1 1
(8.9)
TYPES OF COPOLYMERIZATION
By definition, r1 and r2 represent the relative preference of a given radical that is adding its own monomer
to the other monomer. The physical significance of Equation 8.9 can be illustrated by considering the
product of the reactivity ratios,
r1 r2 =
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k11 k 22
k12 k 21
(8.10)
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COPOLYMERIZATION
The quantity r1r2 represents the ratio of the product of the rate constants for the reaction of a radical
with its own kind of monomer to the product of the rate constants for the cross-reactions. Copolymerization may therefore be classified into three categories depending on whether the quantity r1r2 is unity,
less than unity, or greater than unity.
A. IDEAL COPOLYMERIZATION (r1r2 = 1)
r1 r2 = 1,
then
r1 = 1 r2 or k11 k12 = k 21 k 22
(8.11)
In this case the copolymer equation reduces to
[ ] = r [M ]
[ ] [M ]
d M1
1
d M2
1
(8.12)
2
or
F1 =
r1 f1
rf
= 1 1
f1 ( r1 − 1) + 1 r1 f1 + f2
(8.13)
It is evident that for ideal copolymerization, each radical displays the same preference for adding one
monomer over the other. Also, the end group on the growing chain does not influence the rate of addition.
For the ideal copolymer, the probability of the occurrence of an M1 unit immediately following an M2
unit is the same as locating an M1 unit after another M1 unit. Therefore, the sequence of monomer units
in an ideal copolymer is necessarily random.
The relative amounts of the monomer units in the chain are determined by the reactivities of the
monomer and the feed composition. To illustrate this, we note that the requirement that r1r2 = 1 can be
satisfied under two conditions:
Case 1: r1 > 1 and r2 < 1 or r1 < 1 and r2 > 1. In this case, one of the monomers is more reactive than
the other toward the propagating species. Consequently, the copolymer will contain a greater
proportion of the more reactive monomer in the random sequence of monomer units. An important
practical consequence of ideal copolymerization is that increasing difficulty is experienced in the
production of copolymers with significant quantities of both monomers as the difference in
reactivities of the two monomers increases.
Case 2: r1 = r2 = 1. Under these conditions, the growing radicals cannot distinguish between the two
monomers. The composition of the copolymer is the same as that of the feed and as we said above,
the monomers are arranged randomly along the chain. The copolymer equation becomes:
F1 =
f1 f ⋅
= f1
f1 + f2
(8.14)
B. ALTERNATING COPOLYMERIZATION (r1 = r2 = 0)
When r1 = r2 = 0 (or r1r2 = 0), each radical reacts exclusively with the other monomer; that is neither
radical can regenerate itself. Consequently, the monomer units are arranged alternately along the chain
irrespective of the feed composition. In this case the copolymer reduces to:
[ ] =1
[ ]
d M1
d M2
or
F1 = 0.5
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(8.15)
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POLYMER SCIENCE AND TECHNOLOGY
Table 8.1 Reactivity Ratios of Some Monomers
Monomer 1
Monomer 2
r1
r2
T (°C)
Acrylonitrile
1,3-Butadiene
Methyl methacrylate
Styrene
Vinyl acetate
Vinyl chloride
Methyl methacrylate
Styrene
Vinyl chloride
Styrene
Vinyl acetate
Vinyl chloride
Vinyl acetate
Vinyl chloride
Vinyl chloride
0.02
0.15
0.04
4.2
2.7
0.75
1.35
8.8
0.46
20
10
55
17
0.23
0.30
1.22
0.40
0.05
0.04
0.25
0.58
0.035
0.52
0.015
0.1
0.01
0.02
1.68
40
80
60
50
60
90
50
50
60
60
68
60
60
60
1,3-Butadiene
Methyl methacrylate
Styrene
Vinyl acetate
From Young, L.J., Polymer Handbook, 2nd ed., Brandrup, J. and Immergut, H.H., Eds., John Wiley & Sons, New York, 1975. With permission.
Polymerization continues until one of the monomers is used up and then stops. Perfect alternation occurs
when both r1 and r2 are zero. As the quantity r1r2 approaches zero, there is an increasing tendency toward
alternation. This has practical significance because it enhances the possibility of producing polymers
with appreciable amounts of both monomers from a wider range of feed compositions.
C. BLOCK COPOLYMERIZATION (r1 > 1, r2 > 1)
If r1 and r2 are both greater than unity, then each radical would prefer adding its own monomer. The
addition of the same type of monomer would continue successively until there is a chance addition of
the other type of monomer and the sequence of this monomer is added repeatedly. Thus the resulting
polymer is a block copolymer. In the extreme case of this type of polymerization (r1 = r2 = ∞) both
monomers undergo simultaneous and independent homopolymerization; however, there are no known
cases of this type of polymerization. Even though cases exist where r1r2 approaches 1 (r1r2 = 1), there
are no established cases where r1r2 > 1. Indeed, the product r1r2 is almost always less than unity. Table 8.1
lists the reactivity ratios for some monomers.
Example 8.1: The reactivity ratios for the copolymerization of methyl methacrylate (1) and vinyl
chloride (2) at 68°C are r1 = 10 and r2 = 0.1. To ensure that the copolymer contains an appreciable
quantity (>40% in this case) of the vinyl chloride, a chemist decided to carry out the copolymerization
reaction with a feed composed of 80% vinyl chloride. Will the chemist achieve his objective?
F1 =
2
r1 f12 + f1 f2
10(0.2) + (0.2)(0.8)
=
2
r1 f12 + 2 f1 f2 + r2 f22 10(0.2) + 2(0.2)(0.8) + 0.1(0.8)2
= 0.714
F2 = 1 – F1 = 0.286.*
If the difference in the reactivities of the two monomers is large, it is impossible to increase the proportion
of the less-reactive monomer in the copolymer simply by increasing its composition in the feed.
IV.
POLYMER COMPOSITION VARIATION WITH FEED CONVERSION
Since the reactivity ratios r1 and r2 are generally of different magnitudes, there are necessarily differences
in the rate of possible growth reactions. Consequently, the composition of the feed, f1, and that of the
* Although f2 = 80%, F2 = 28.6%.
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215
COPOLYMERIZATION
Figure 8.1 Variation of instantaneous composition of copolymer (mole fraction, F1) with feed composition (mole
fraction f1 for ideal copolymerization with the values of r1 = 1/r2 indicated).
polymer, F1, is neither equal nor constant throughout the polymerization. Therefore, the polymer produced
over a finite range of conversion consists of a summation of increments of polymer differing in composition. Consider, for example, the copolymerization of styrene (1) and vinyl acetate (2). The reactivity
ratios are r1 = 55 and r2 = 0.015. This means that either radical has a much greater preference to add
styrene than vinyl acetate. The first polymer formed consists mainly of styrene. This also means a faster
depletion of styrene in the feed. As polymerization proceeds, styrene is essentially used up and the last
polymer formed consists mostly of vinyl acetate. However, at 100% conversion, the overall polymer
composition must reflect the initial composition of the feed. It follows that the copolymers generally
have a heterogeneous composition except in very special cases.
Equations 8.5 and 8.9 give the instantaneous polymer composition as a function of feed composition
for various reactivity ratios. Figure 8.1 shows a series of curves calculated from these equations for ideal
copolymerization. The range of feed composition that gives copolymers containing appreciable amounts
of both monomers is small except the monomers have very similar reactivities.
The curves for several nonideal cases are shown in Figure 8.2. These curves illustrate the effect of
increasing tendency toward alternation. With increasing alternation, a wider feed composition yields a
copolymer containing substantial amounts of each monomer. This tendency is utilized in practice for
the preparation of many important copolymers.
In those cases, where r1 and r2 are both either less or greater than unity, the curves of Figure 8.2 cross
the line F1 = f1. The points of interception represent the occurrence of azeotropic copolymerization; that
is, polymerization proceeds without a change in the composition of either the feed or the copolymer.
For azeotropic copolymerization the solution to Equation 8.5 with d[M1]/d[M2] = [M1]/[M2] gives the
critical composition.
[M ] = 1 − r
[M ] 1 − r
1
2
2
1
(8.16)
(f1 )c = 2 −1 −r r−2 r
1
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2
(8.17)
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POLYMER SCIENCE AND TECHNOLOGY
Figure 8.2 Instantaneous composition of copolymer F1 as a function of monomer composition f1 for the values
of the reactivity ratios r1/r2 indicated.
If both r1 and r2 are greater than unity or if both are less than unity, then (f1)C lies within the acceptable
range 0 < f1 < 1. If r1 > 1 and r2 < 1 or r1 < 1 and r2 > 1, then there will be no critical feed composition,
(f1)C. As we said earlier, cases where both r1 and r2 are greater than unity are not known; whereas, there
are numerous cases where r1 and r2 are less than unity. When r1 @1 and r2 !1 or r1 !1 and r2 @1, the two
monomers have a tendency to polymerize consecutively; the first polymer is composed predominantly
of the more reactive monomer; the other monomer polymerizes only after almost all of the more reactive
monomer has been exhausted. The case of styrene–vinyl acetate discussed above exemplifies this type
of copolymerization.
Our discussion thus far has indicated that during copolymerization, the composition of both the feed
and the polymer vary with conversion. To follow this composition drift, it is necessary to integrate the
copolymer equation — a problem that is complex. Consider a system that is composed initially of M
total moles of the two monomers (M = M1 + M2) and in which the resulting copolymer is richer in M1
than the feed (F1 > f1). When dM moles have been polymerized, the polymer will contain F1 dM moles
of M1 while the feed content of M1 will be reduced to (M – dM) (f1 – df1) moles. Writing a material
balance for M1:
Mf1 − (M − dM) (f2 − df1 ) = F1 dM
(8.18)
Expanding and neglecting the second-order differential yields:
dM M = df1 (F1 − f2 )
(8.19)
On integration, Equation 8.19 becomes
ln M M o =
Copyright 2000 by CRC Press LLC
∫
f
f10
df1 (F1 − f1 )
(8.20)
217
COPOLYMERIZATION
where M0 and f10 are the initial values of M and f1. Using Equation 8.19, it is possible to calculate F1
by choosing values for f1, and the integral may be evaluated graphically or numerically. This gives the
relation between feed composition and the degree of conversion (1 – M/M0). An analytic solution to
Equations 8.19 and 8.20 has been obtained. For r1 ≠ 1 and r2 ≠ 1,
f
M
= 1
M0
(f1 ) 0
α
f
2
(f2 ) 0
β
(f1 ) − δ
0
f1 − σ
γ
(8.21)
where the superscripts are given by:
α=
r2
r
1 − r1 r2
; β= 1 ; γ =
1 − r2
1 − r1
1
−
( r1 ) (1 − r2 )
1 − r2
δ=
2 − r2 − r2
(8.22)
The feed composition variation with conversion gives the instantaneous composition of the copolymer
as a function of conversion. It is also useful to know the overall average composition <F1> for a given
conversion. This can be obtained through a material balance, say, for M1 in a batch reactor. Moles M1
in feed = moles of M1 in polymer + moles of unreacted M1.
(f1 )0 M0 =
F1 (M 0 − M) + f1 M
(8.23)
f10 − f1 (M M 0 )
(8.24)
On rearrangement Equation 8.23 becomes:
F =
(1 − M M )
0
Example 8.2: What is the composition of the copolymer formed by the polymerization of an equimolar
mixture of butadiene (1) and styrene (2) at 66°C? Which will contain more styrene — the polymer
formed first or that formed later in the reaction?
Solution: From Table 8.1, r1 = 1.35 and r2 = 0.58
F1 =
r1 f12 + f1 f2
r1 f12 + 2 f1 f2 + r2 f22
1.35 (0.5) + (0.5)
2
2
2
1.35 (0.5) + 2 (0.5) + 0.58 (0.5)
2
=
2
= 0.60
k11 = 1.35 k12 , k 22 = 0.58 k 21
The rate of consumption of butadiene by either radical is greater than the rate of addition of styrene.
Therefore the polymer formed first will be richer in butadiene while the polymer formed at the later
stages of the reaction will be richer in styrene.
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218
POLYMER SCIENCE AND TECHNOLOGY
Example 8.3: Estimate the feed and copolymer compositions for the azeotropic copolymerization of
acrylonitrile and styrene at 60°C.
Solution: For azeotropic copolymerization:
f1c =
1 − r2
2 − r1 − r2
f1c =
1 − 0. 4
2 − 0.04 − 0.4
From Table 8.1
f1 =
=
r1 f12 + f1 f2
r f + 2 f1 f2 + r2 f22
2
1 1
0.04 (0.38) + (0.38) (0.62)
2
2
0.04 (0.38) = 2 (0.38) (0.62) + 0.4 (0.62)
2
= 0.38
Example 8.4: Plot graphs showing the variation of the instantaneous copolymer composition F1 with
monomer composition for the following systems:
I. Vinyl acetate (1), maleic anhydride (2), 75°C, r1 = 0.055, r2 = 0.003.
II. Styrene (1), vinyl acetate (2), 60°C, r1 = 55, r2 = 0.01.
III. Vinyl chloride (1), methyl methacrylate (2) 68°C, r1 = 0.1, r2 = 10
Comment on the shapes of the curves.
Solution:
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219
COPOLYMERIZATION
The range of feed composition that gives copolymers with appreciable quantities of both monomers is
small for Cases II and III where the differences in reactivity ratios are large. However, in Case I where
the reactivity ratios are comparable, copolymers with comparable quantities of both monomers are
obtainable.
V. CHEMISTRY OF COPOLYMERIZATION
By definition, monomer reactivity ratios are independent of the initiation and termination steps of
copolymerization reaction. In addition, they are virtually independent of the reaction medium, and their
dependence on temperature is minimal. The relative reactivities of a series of monomers are determined
by the reactivity of the individual monomer and that of the attacking radical. The reactivities of monomers
and radicals are themselves dependent on the nature of the substituents on the monomer double bond.
The influence of substituents on reactivity is threefold:
• Enhancement of monomer reactivity through activation of the double bond
• Conferment of resonance stabilization on the resulting radical
• Provision of steric hindrance at the reaction site
We now briefly discuss the contribution of these factors to radical copolymerization reactions.
A. MONOMER REACTIVITY
The relative reactivities of monomers to a given radical can be obtained from analysis of the relative
reactivity ratios. This can be seen by considering the inverse of the monomer reactivity ratio:
1 r1 = k12 k11
(8.25)
The inverse of the reactivity ratio (Equation 8.25) is the ratio of the rate of reaction of the given radical
with another monomer to its rate of reaction with its own monomer. If the rate of reaction of the reference
radical with its own monomer is taken as unity (i.e., k11 = 1), then the resulting k12 values give the
relative reactivities of monomers with respect to the reference radical. A list of such values is given in
Table 8.2.
Note that a different reference is taken for each column and as such only values in each column can
be compared; horizontal comparisons are meaningless. In general, the relative reactivities of monomers
decrease from the top to the bottom of each column; the order of decrease is irregular due to the specificity
of addition of monomers by radicals. It is evident that the effect of substituents in enhancing monomer
reactivity is in the order:
C6H5
CH
CH2
COCH3
CN
COOR
Cl
CH2Y
OCOCH3
OR
(Str. 1)
The effect of a second 1-substituent is generally additive.
Table 8.2 Relative Reactivities of Monomers to Reference Radicals at 60°C
Reference Radical
Monomer
Styrene
Methyl methacrylate
Acrylonitrile
Vinylidene chloride
Vinyl chloride
Vinyl acetate
Styrene
Methyl Methacrylate
Acrylonitrile
Vinyl Chloride
Vinyl Acetate
(1.0)
1.9
2.5
5.4
0.059
0.019
2.2
(1.0)
0.82
0.39
0.10
0.05
25
6.7
(1.0)
1.1
0.37
0.24
50
10
25
5
(1.0)
0.59
100
67
20
10
4.4
(1.0)
From Billmeyer, F.W., Jr., Textbook of Polymer Science, 2nd ed., Interscience, New York, 1971. © John Wiley & Sons.
Reprinted with permission of John Wiley & Sons.
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220
POLYMER SCIENCE AND TECHNOLOGY
Table 8.3 Radical–Monomer Propagation Rate Constants at 60°C (l/mol-s)
Radical
Monomer
Butadiene
Styrene
Methyl
Methacrylate
Methyl
Acrylate
Vinyl
Chloride
Vinyl
Acetate
100
74
134
132
11
—
250
143
278
206
8
2.6
2,820
1,520
705
370
70
35
42,000
14,000
4,100
2,090
230
520
350,000
600,000
123,000
200,000
12,300
7,300
—
230,000
150,000
23,000
10,000
2,300
Butadiene
Styrene
Methyl methacrylate
Methyl acrylate
Vinyl chloride
Vinyl acetate
From Billmeyer, F.W., Jr., Textbook of Polymer Science, 2nd ed., Interscience, New York, 1971. © John Wiley
& Sons. Reproduced with permission of John Wiley & Sons.
The above order of monomer reactivities corresponds to the order of increased resonance stabilization
of the resulting radical by the particular substituent. Substituents with unsaturation leading to a monomer
with conjugated bonds are most effective in conferring resonance stabilization on radicals. On the other
hand, substituents such as chlorine that have only nonbonding electrons for interaction with the radical
show only weak stabilization. In the case of styrene, for example, the resonance stabilization energy is
about 20 kcal/mol compared with 1 to 4 kcal/mol for the unconjugated systems. It is important to
remember, however, that substituents that stabilize the product radical also stabilize the monomer, albeit
to a much lesser extent. Again, using styrene as an example, the monomer is stabilized to about 3 kcal/mol.
Therefore a limited compensation is derived from the stabilization of monomers by substituents.
B. RADICAL REACTIVITY
The relative reactivities of radicals to a reference monomer can be obtained from the product of the
inverse of the reactivity ratio (1/r1 = k12/k11) and the appropriate propagation rate constants for homopolymerization (k11). Some values are listed in Table 8.3.
The order of enhancement of radical reactivity due to substituents is the reverse of that for the
monomers. This should be expected because the enhancement of monomer reactivity by a substituent
is due to its stabilization and the consequent decrease of the reactivity of the corresponding radical. The
degree of depression of radical reactivity by substituents turns out to be much greater than the extent
of enhancement of monomer reactivity. The styrene radical is about 1000 times less reactive than the
vinyl acetate radical to a given monomer if the effects of alternation are disregarded, but then the styrene
monomer is only about 50 times more reactive than the vinyl acetate to a given radical.
C. STERIC EFFECTS
Monomer–radical reaction rates are also influenced by steric hindrance. The effect of steric hindrance
in reducing monomer reactivity can be illustrated by considering the copolymerization reaction rate
constants (k12) for di- and tri-substituted ethylene. Table 8.4 lists some of these values.
Table 8.4 Rate Constants (k12) for Radical–Monomer Reactions
Polymer Radical
Monomer
Vinyl chloride
Vinylidene chloride
cis-1,2-dichloroethylene
trans-1,2-dichloroethylene
Trichloroethylene
Tetrachloroethylene
Vinyl acetate
Styrene
Acrylonitrile
10,000
23,000
370
2,300
3,450
460
8.7
78
0.60
3.9
8.6
0.70
720
2,200
—
—
29
4.1
From Odian, G., Principles of Polymerization, McGraw-Hill, New York, 1970.
With permission.
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221
COPOLYMERIZATION
We have already alluded to the fact that the addition of a second substituent to the 1- or α-position
increases monomer reactivity. However, when the same substituent is in the 2- or β-position (i.e., 1,2disubstitution), the reactivity of the monomer decreases 2- to 20-fold. This has been attributed to the
resulting steric hindrance between the substituent and the attacking radical. The role of steric hindrance
in the reduction of the reactivity of 1,2-disubstituted vinyl monomers can be further illustrated by the
fact that while these monomers undergo copolymerization with other monomers, say, styrene, they exhibit
extreme reluctance to homopolymerize. Homopolymerization is prevented because of the steric hindrance
between a 2-substituent on the attacking radical and the monomer. On the other hand, there is no 2- or
β-substituent on the attacking styrene radical; consequently, copolymerization is possible.
Example 8.5: Arrange the following monomers in the possible order of decreasing reactivity with an
acrylonitrile radical. What is the basis of your arrangement?
a.
b.
c.
d.
Methacrylate
Vinyl methyl ether
Methyl methacrylate
Vinyl acetate
Solution:
Monomer
a. Methacrylate
Structure
CH2
CH
C
O
O
CH3
b. Vinyl methylether
CH2
CH
O
CH3
c. Methyl methacrylate
CH3
CH3
C
C
O
O
CH3
d. Vinyl acetate
CH2
CH
O
C
O
CH3
Order of reactivity: c > a > d > b. Reason: (1) increasing stabilization of resulting radical; (2) second
substituent in 1-position increases monomer reactivity.
D. ALTERNATION-POLAR EFFECTS
We indicated in our earlier discussion that the relative reactivities of different monomers were not exactly
the same for different radicals. Similarly, the order of reactivity of different radicals is influenced by the
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POLYMER SCIENCE AND TECHNOLOGY
Table 8.5 Product of Reactivity Ratios of Monomers Showing Order
of Alternating Tendency
Vinyl acetate
—
Butadiene
0.55
0.78
0.39
0.31
0.30
0.19
0.60
0.10
0.90
0.04
0.21
0.006
0.004
—
Styrene
0.34
0.24
0.16
0.11
0.016
0.02
Vinyl chloride
1.0
Methyl methacrylate
0.96
0.61
Vinylidene chloride
0.24
0.96
0.08
Methyl acrylate
0.11
0.18
0.34
0.84
Acrylonitrile
0.06
—
0.56
—
—
Diethyl fumarate
reference monomer. This suggests, therefore, that monomer reactivity is a function of radical reactivity
and vice versa. A close examination of monomer reactivity data reveals a tendency toward enhanced
copolymerization reactivities for certain pairs of monomers. This is evidently a result of some monomer–radical affinity. This tendency toward enhanced reactivity for certain monomer pairs is a general
phenomenon in radical copolymerization and is due to the alternating tendency in comonomer pairs. As
the quantity r1r2 deviates from unity (ideal behavior) and approaches zero, the tendency for alternation
increases. It is possible to arrange monomers in order of their r1r2 values such that the farther apart any
two pairs of monomers are, the greater their tendency toward alternation. Table 8.5 shows such an
arrangement. Notice, however, that there some exceptions, presumably due to the predominance of steric
factors; for example, vinyl chloride and styrene show a greater alternation tendency than vinyl chloride
and vinyl acetate, even though the former monomer pair is closer in Table 8.5 than the latter.
The order of occurrence of monomers in Table 8.5 is obviously a reflection of the polarity of the
double bond. Observe that the product r1r2 approaches unity only in those cases where the substituents
on the monomers are either both electron-donating or electron-withdrawing substituents. Expressed
differently, alternation tendency increases if the substituents on both monomers exhibit different electrondonating or electron-attracting characteristics. Alternation tendency is enhanced by an increased difference in the polarity of monomer pairs.
Example 8.6: Which of the following pairs of monomers will most probably form an alternating
copolymer?
a. Butadiene (1), Styrene (2), 60°C, r1 = 1.89, r2 = 0.78.
b. Vinyl acetate (1), styrene (2), 60°C, r1 = 0.01, r2 = 55.
c. Maleic anhydride (1), isopropenyl acetate (2), 60°C, r1 = 0.002, r2 = 0.032.
Solution:
Monomer
r1 r2
(a)
(b)
(c)
1.4742
0.5500
0.000064
Alternating tendency increases generally as r1r2 approaches zero. Hence monomer pair (c) has the highest
alternating tendency.
VI.
THE Q-e SCHEME
The Q-e scheme is an attempt to express free radical copolymerization data on a quantitative basis by
separating reactivity ratio data for monomer pairs into parameters characteristic of each monomer. Under
this scheme, radical–monomer reaction rate constant k12 is written as:
k12 = P1 Q 2 exp (− e1e 2 )
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(8.26)
223
COPOLYMERIZATION
Table 8.6 Q and e Values for Monomers
Monomer
t-Butyl vinyl ether
Ethyl vinyl ether
Butadiene
Styrene
Vinyl acetate
Vinyl chloride
Vinylidene chloride
Methyl methacrylate
Methyl acrylate
Methyl vinyl ketone
Acrylonitrile
Diethyl fumarate
Maleic anhydride
e
Q
–1.58
–1.17
–1.05
–0.80
–0.22
0.20
0.36
0.40
0.60
0.68
1.20
1.25
2.25
0.15
0.032
2.39
1.00
0.026
0.44
0.22
0.74
0.42
0.69
0.60
0.61
0.23
From Alfrey, T., Jr. and Price, C.C., J. Polym. Sci., 2,
101, 1947. With permission.
where P1 represents the reactivity of radical M1· and Q2 the reactivity of monomer M2; e1 and e2 represent
the degrees of polarity of the radical and monomer, respectively. Both P and Q are determined by the
resonance characteristics of the radical and monomer. It is assumed that both the monomer and its radical
have the same e value. Consequently,
( )
k11 = P1 Q1 exp − e12
(8.27)
Therefore
[
]
(8.28)
[
]
(8.29)
r1 = k11 k12 = Q1 Q 2 exp − e1 (e1 − e 2 )
Similarly
r2 = k 22 k 21 = Q 2 Q1 exp − e 2 (e 2 − e1 )
Thus, according to the Q-e scheme, by assigning values to Q and e, it should be possible to evaluate r1
and r2 for any monomer pair. A selected list of Q and e values is shown in Table 8.6. Negative values
of e indicate electron-rich monomers, while positive e values indicate electron-poor monomers.
The Q-e scheme is subject to criticisms. First, there seems to be no justification for assuming the
same e values for the monomer and the radical derived from it. Second, the Q and e values for a particular
monomer are not unique; they vary with the monomer to which the monomer is paired (Table 8.7). In
spite of these flaws, however, the Q-e scheme provides a semi-empirical basis for correlating the effect
of structure on monomer reactivity.
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POLYMER SCIENCE AND TECHNOLOGY
Table 8.7 Variation in Q and e Values
Monomer (M1)
Comonomer
e1
Q1
Acrylonitrile
Styrene
Vinyl acetate
Vinyl acetate
Vinyl chloride
Vinyl chloride
Styrene
Methyl acrylate
1.20
0.90
1.0
1.3
1.6
0.2
0.0
0.44
0.37
0.67
0.37
0.37
0.024
0.035
Vinyl chloride
From Odian, G., Principles of Polymerization,
McGraw-Hill, New York, 1970. With permission.
VII.
PROBLEMS
8.1. Styrene can undergo copolymerization reaction with either methyl methacrylate or vinyl chloride. It has
been suggested that the resulting copolymers become brittle for styrene composition greater than 70%.
Explain why even with a feed composition as low as 25% styrene, styrene–vinyl chloride copolymer is
brittle, while styrene–methyl methacrylate copolymer is not.
8.2. Discuss the composition and structure of the polymer product obtained from the reaction of acrylonitrile
(1) and 1,3-butadiene (2) in the molar ratios 25:75, 50:50, and 75:25. r1 = 0.02, r2 = 0.3 at 40°C.
8.3. Methyl methacrylate (1) and vinyl chloride (2) form an ideal copolymerization at 68°C. What is the
composition of this copolymer for a feed composition f1 = 0.75 and r2 = 0.1?
8.4. For a particular application, it has been established that an alternating copolymer is most suitable. Given
the following monomers — butadiene, styrene, acrylonitrile, and vinyl chloride — which monomer pair
would be best studied for the application?
8.5. Arrange the following monomers in increasing order of reactivity with a vinyl acetate radical. Explain
the basis of your arrangement.
a.
b.
c.
d.
e.
Acrylonitrile
Styrene
α-Methyl styrene
2-Methyl styrene
Vinyl ethyl ether
8.6. Maleic anhydride does not homopolymerize, but will readily form alternating copolymer with styrene.
Explain.
8.7. The following are the Q and e values for monomers A, B, C, and D:
Monomer
Q
e
A
B
C
D
2.39
0.15
0.69
0.42
–1.05
–1.58
0.68
0.60
Monomers A and B; A and C; and C and D are subjected, respectively, to copolymerization reactions.
Which of these reactions will most likely result in an alternating polymer? What is the composition of
the copolymer formed at low conversion from equimolar mixtures of the pairs of monomers?
8.8. Describe the probable proportions and sequences of monomers entering a copolymer chain at the
beginning of polymerization of a 1:1 molar mixture of each of the following pairs:
a. Vinyl acetate (1) and isopropenyl acetate (2); r1 = r2 = 1
b. Butadiene (1) and styrene (2); r1 = 1.89, r2 = 0.78
c. Maleic anhydride (1) and stilbene (2); r1 = r2 = 0.01
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225
COPOLYMERIZATION
d. Maleic anhydride (1) and isopropenyl acetate (2); r1 = 0.002, r2 = 0.032
In each case, would the composition of polymer formed toward the end of the reaction (where the
monomers are nearly consumed) be much different, and if so how?
REFERENCES
1. Young, L.J., Copolymerization reactivity ratios, in Polymer Handbook, 2nd ed., Brandrup, J. and Immergut, H.H.,
Eds., John Wiley & Sons, New York, 1975.
2. Billmeyer, F.W., Jr., Textbook of Polymer Science, 2nd ed., Interscience, New York, 1971.
3. Odian, G., Principles of Polymerization, McGraw-Hill, New York, 1970.
4. Young, L.J., Tabulation of Q-e values, in Polymer Handbook, 2nd ed., Brandrup, J. and Immergut, E.H., Eds., John
Wiley & Sons, New York, 1975.
5. Alfrey, T., Jr. and Price, C.C., J. Polym. Sci., 2, 101, 1947.
6. Mayo, F.R. and Walling, C., Chem. Rev., 46, 191, 1950.
7. Kruse, R.K., J. Polym. Sci. Part B, 5, 437, 1967.
8. Tidwell, P.W. and Mortimer, G.A., J. Macromol. Sci. Rev., C4, 281, 1970.
9. Williams, D.J., Polymer Science and Engineering, Prentice-Hall, Englewood Cliffs, NJ, 1971.
10. Flory, P.J., Principles of Polymer Chemistry, Cornell University Press, Ithaca, NY, 1953.
Copyright 2000 by CRC Press LLC
Chapter 9
Polymer Additives and Reinforcements
I.
INTRODUCTION
In Chapter 5, we discussed the techniques for the modification of polymer properties based on structural
modification of polymers either during or post polymerization. Even though structural modification of
polymers often leads to significant property modification, very few polymers are used technologically
in their chemically pure form; it is generally necessary to modify their behavior by the incorporation of
additives. Additives are usually required to impart stability against the degradative effects of various
kinds of aging processes and enhance product quality and performance. Thus many commercial polymers
must incorporate thermal and light stabilizers, antioxidants, and flame retardants.1 In addition to these
additives that influence essentially the chemical interaction of polymers with the environment, other
additives are usually employed to reduce costs, improve aesthetic qualities, or modify the processing,
mechanical, and physical behavior of a polymer. Such additives include plasticizers, lubricants, impact
modifiers, antistatic agents, pigments, and dyes. These additives are normally used in relatively small
quantities; however, nonreinforcing fillers are employed in large quantities to reduce overall formulation
costs provided this does not result in significant or undesirable reduction in product quality or performance. In some cases, a given polymer may still not meet the requirements of a specific application
even with the incorporation of additives. In such cases, the desired objective may be achieved through
alloy formation or blending of two or more polymers. In this chapter, we discuss the upgrading of the
performance of polymers through the use of additives and reinforcements.
II.
PLASTICIZERS
Many commercial polymers such as cellulosics, acrylics, and vinyls have glass transition temperatures,
Tg, above room temperature. They are therefore hard, brittle, glasslike solids at ambient temperatures.
To extend the utility of these polymers, it is usually necessary to reduce the Tg to below the anticipated
end-use temperature. The principal function of a plasticizer is to reduce the Tg of a polymer so as to
enhance its flexibility over expected temperatures of application. For example, unplasticized PVC is a
rigid, hard solid used in such applications as credit cards, plastic pipes, and home siding. Addition of
plasticizers such as phthalate esters reduces the modulus and converts the polymer into a leathery material
used in the manufacture of upholstery, electrical insulation, and similar items.
Plasticizers are usually high boiling organic liquids or low melting solids. They are also sometimes
moderate-molecular-weight polymers. Like ordinary solvents, plasticizers act through a varying degree
of solvating action on the polymer. The plasticizer molecules are inserted between the polymer molecules
thereby pushing them apart. This reduces the intensity of the intermolecular cohesive forces. The
plasticizer may also depend on polar intermolecular attraction between the plasticizer and polymer
molecules, which effectively nullifies dipole–dipole interactions between polymer molecules. As a result,
plasticization is difficult to achieve in nonpolar polymers like polyolefins and highly crystalline polymers.
Polymer plasticization can be achieved either through internal or external incorporation of the
plasticizer into the polymer. Internal plasticization involves copolymerization of the monomers of the
desired polymer and that of the plasticizer so that the plasticizer is an integral part of the polymer chain.
In this case, the plasticizer is usually a polymer with a low Tg. The most widely used internal plasticizer
monomers are vinyl acetate and vinylidene chloride. External plasticizers are those incorporated into
the resin as an external additive. Typical low-molecular-weight external plasticizers for PVC are esters
formed from the reaction of acids or acid anhydrides with alcohols. The acids include ortho- and isoor terephthalic, benzoic, and trimellitic acids, which are cyclic; or adipic, azeleic, sebacic, and phosphoric
acids, which are linear. The alcohol may be monohydric such as 2-ethylhexanol, butanol, or isononyl
alcohol or polyhydric such as ethylene or propylene glycol. The structures of some plasticizers of PVC
are shown in Table 9.1.
0-8493-????-?/97/$0.00+$.50
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228
POLYMER SCIENCE AND TECHNOLOGY
Table 9.1 Chemical Structures of Some PVC Plasticizers
Plasticizer Type
Chemical Structure
Example
O
Phthalate Esters
(Dialkylphthalate)
O
CH3
C
OR
C
O
C
OR
C
O
CH3
CH3
CH3
O
Di-2-ethylhexyl phthalate or
Dioctylphthalate (DOP)
O
Phosphate Esters
(Trialkyl-phosphate)
RO
CH3
O
O
P
O
OR
OR
P
O
O
CH3
CH3
Tricresyl phosphate (TCP)
O
Adepates, azelates,
oleates, sebacates
(Aliphatic diester)
RO
O
C
(CH2)n
C
CH3
OR
O
C
CH3
O
(CH2)4
C
CH3
O
Di-2-ethylhexyl adepate (DOA)
O
Glycol Derivatives
O
CH3
R
O
C
O
(CH2)n
O
C
O
R
C
CH3
O
CH2
CH3
CHO
CH2 CHO
O
C
Dipropyleneglycol benzoate
O
Trimellitates
(Trialkyltrimellitate)
O
C
OR
C
OR
C
O
RO
C
O
O
CH3
C
CH3
O
C
O
CH3
CH3
O
O
CH3
CH3
Trisethylhexy trimellitate (TOTM)
Phthalate, terephthalate, adipate, and phosphate esters are generally referred to as monomeric plasticizers. Linear polyesters obtained from the reaction of dibasic acids such as adipic, sebacic, and azelaic
acids with a polyol constitute the group of polymeric or permanent plasticizers. They have much higher
molecular weights than the monomerics and as such are less volatile when exposed to high temperatures
either during processing or in end-use situations, less susceptible to migration, and less extractible.
They impart enhanced durability or permanence on PVC products, hence the name permanent
plasticizers. Another group of plasticizers known as epoxy plasticizers is derived from vegetable oils.
An example is epoxidized soybean. These plasticizers confer heat and light stability on PVC products,
which, however, usually have relatively poor low-temperature properties.
The ideal plasticizer must satisfy three principal requirements. These are compatibility, performance,
and efficiency. In addition, it should be odorless, tasteless, nontoxic, nonflammable and heat stable.
Compactibility of a plasticizer with the host polymer demands the absence of blooming even with long
usage of the plasticized material. Incompatibility can also be manifested by poor physical properties,
possibly after some period of usage.
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POLYMER ADDITIVES AND REINFORCEMENTS
229
Permanence requires low volatility, extractability, nonmigration, and heat and light stability. Lack of
permanence involves long-term diffusion into the environment. The consequent loss of plasticizer
gradually enhances brittleness as the Tg of the plasticized polymer increases. Volatility is generally a
function of molecular weight. Increasing the molecular weight of the plasticizer by using polymeric
plasticizers tends to decrease volatility and hence increase permanence. However, this may lead to a
decrease in low-temperature flexibility. Internal plasticization precludes plasticizer migration or volatility
since the plasticizer molecules are an integral part of the polymer chain.
The level of plasticizer required to achieve the desired changes in properties is a measure of plasticizer
efficiency. Plasticizer efficiency may also be measured on the basis of the magnitude of change induced
in a number of physical properties of the polymer such as tensile strength, modulus, or hardness. For
example, the actual reduction in Tg of the polymer per unit weight of plasticizer added is also known
as the plasticizer efficiency.
It must be emphasized, however, that quite frequently no single plasticizer can satisfy all the above
requirements or produce all the desired property enhancements. It is generally necessary to blend several
plasticizers and compromise some properties, particularly those that are not critical to the specific
application. For example, some applications require that DOP-plasticized PVC remains flexible at low
temperatures. This requires further addition of DOP to the formulation to achieve the desired flexibility,
but the additional DOP would adversely affect the hardness and performance of the product at ambient
temperatures. Instead, aliphatic diesters such as DOA and DBS (dibutyl sebacate), which are more
effective in enhancing low-temperature flexibility than DOA, are needed to improve low-temperature
performance. However, the use of these esters is accompanied by unacceptable levels of volatility and
oil extraction. Consequently, in such applications a combination of phthalate and aliphatic esters is
required to produce the desired product even though some compromise in product performance will occur.
Plasticizers, particularly for PVC, constitute one of the largest segments of the additives market. The
wide range of applications of flexible PVC depends largely on broad plasticizer technology that imparts
the desired flexibility. The characteristic properties and typical applications of plasticized PVC are shown
in Table 9.2.
III.
FILLERS AND REINFORCEMENTS (COMPOSITES)
The need to meet exacting end-use requirements and at the same time reduce costs is stimulating a broad
spectrum of product development involving the use of fillers and reinforcements to upgrade product
performance rather than the development of new and usually more expensive resins. For example, because
of their advantageous light weight, high strength, fatigue life, and corrosion resistance, structural composites have been used successfully and admirably in aircraft and in numerous industrial and consumer
applications in place of conventional materials like metals. Fiber-reinforced materials have moved within
a short time from being a curiosity to having a central role in engineering materials development.
Polymers, thermoplastics, and thermosets can be reinforced to produce quite frequently a completely
new kind of structural materials.
Different types of fillers are employed in resin formulations; the most common are calcium carbonate,
talc, silica, wollastonite, clay, calcium sulfate, mica, glass structures, and alumina trihydrate. Fillers
serve a number of purposes. Inert materials like wood flour, clay, and talc serve to reduce resin costs
and, to a certain extent, improve processability and heat dissipation in thermosetting resins. Both alumina
trihydrate and talc improve flame retardance. Mica is used to modify the electrical and heat-insulating
properties of a polymer. Parts molded from composites containing phlogopite mica as a reinforcement
exhibit little or no warpage on demolding or subjection to elevated temperature. A variety of fillers, e.g.,
particulate fillers like carbon black, aluminum flakes, and metal or metal-coated fibers may be used to
reduce mold shrinkage as well as to produce statically conductive polymers, shielding of electromagnetic
interference/radio frequency interference (EMI/FMI).4 Particulate fillers such as carbon black or silica
are used as reinforcing fillers to improve the strength and abrasion resistance of commercial elastomers.
Fibers such as asbestos, glass, carbon, cellulosics, and aramid are used principally to improve some
mechanical property/properties such as modulus, tensile strength, tear strength, abrasion resistance,
notched impact strength, and fatigue strength as well as enhance the heat-deflection temperature.
Fibers are available in a variety of forms. For example, carbon fibers are obtained from the pyrolysis
of organic materials such as polyacrylonitrile (PAN) and rayon for long fibers and pitch for short fibers.
Copyright 2000 by CRC Press LLC
230
Table 9.2
POLYMER SCIENCE AND TECHNOLOGY
General-Purpose PVC Plasticizers
Abbreviations
Chemical
Designations
BBP
Butyl benzyl
phthalate
BOP
Butyl octyl
phthalate
Dihexyl phthalate
Diisoheptal
phthalate
DHP
DIHP
DOP (DEHP)
Di-2-ethylhexyl
phthalate
DINP
Diisononyl
phthalate
Linear
phthalates
C7,C9,C11,
C6,C8,C10
DIDP
Diisodecyl
phthalate
DUP
Diundecyl
phthalate
DTOP
Ditridecyl
phthalate
UDP
Undecyl dodecyl
phthalate
Copyright 2000 by CRC Press LLC
Advantages
Limitations
Low-Molecular-Weight Phthalates
Rapid fusing (high-solvating);
High volatility as
low migration into flooring
compared with the
asphaltics
commodity phthalates;
costs slightly more than
DOP
Rapid fusing
Same as above
Same as above
Same as above
Same as above
Same as above
Commodity Phthalates
Good property balance; industry Moderate plasticizer
standard for general
volatility; moderate
applications; relatively low
low-temperature
cost; good weatherability; good
properties (–38°C at
compatibility in PVC)
40% concentration in
PVC).
Low volatility (less than 2% per Fair weatherability; fair
ASTM D1203); good electrical
low-temperature
properties; similar to DOP in
properties (similar to
cost
that of DOP)
Low volatility (–2%); better
Costs slightly more than
low-temperature properties:
DOP; slightly poorer
7 to 9°C better than DOP and
electrical properties
DIMP (to –47°C at 40%
plasticizer concentration); very
good weathering
Very low volatility (–1%); good Low-temperature
electrical properties; good
properties (–38°C); fair
viscosity stability
weatherability
High-Molecular-Weight Specialties
Good electrical properties;
Slow processing
volatility less than 1% at 48°C
characteristics;
to –50°C at 40% concentration;
marginal compatibility
exceptionally good viscosity
with PVC; costs about
stability in a plastisol; very low
20 cents per pound
fogging in interior applications
more than DOP; high
(automotive)
hydrocarbon extraction
Less than 1% volatility; good
Slow processor; high
electrical properties
viscosity; fair lowtemperature properties
(–38°C); marginal
compatibility with
PVC
Volatility less than 1%
Fair low-temperature
properties to –38°C;
slow processor;
marginal compatibility
with PVC
Applications
Flooring: processing aid
in calendering and
extrusion; expandedfoam formulations
Same as above
Same as above
Same as above
General-purpose
calendering and
extrusion; plastisols;
flooring
Competes “head on”
with DOP
Automotive wire and
cable jacketing
compounds; outdoor
applications; plastisols;
automotive interiors
(except for crash pads)
Wire and cable; sealants;
plastisols; calendering
applications
Wire and cable up to
90°C; sealants;
automotive interiors
(crash pads); roofing
membranes
Wire and cable up to
90°C; automotive
interiors; sealants
Wire and cable up to
90°C; automotive
interiors
231
POLYMER ADDITIVES AND REINFORCEMENTS
Table 9.2 (continued)
Abbreviations
Linear
phthalates
General-Purpose PVC Plasticizers
Chemical
Designations
C9,C10,C11
Advantages
Limitations
Volatility less than 1%; lowtemperature properties to 50°C;
good outdoor weathering; low
viscosity; improved
compatibility with PVC
Applications
Costs more than DOP
Wire and cable to 90°C;
automotive interiors;
roofing membranes
Note: Plasticizers for PVC also include special-purpose products in applications such as flooring (benzyl phthalates); stain
resistance (monobenzoates or benzyl phthalates); food and film wrap (adipate esters with superior low-temperature and
oxygen-permeation properties); flame retardance (phosphate esters); wire and cable (trimellitates with exceptionally low
volatility); heat stabilizers as well as plasticizers (epoxy); mar resistance of styrene (polymerics, isophthalates, and
terephthalates); and polymeric plasticizers for long-term usage. These materials are more expensive than DOP and have
specialty niches in the market.
From Wigotsky, V., Plast. Eng., 40(12), 19, 1984. With permission.
Glass structures, the most widely used reinforcement, are available as roving, mat, hollow or solid
spheres, bubbles, long or short fibers, and continuous fibers.5 The form has a significant influence on
properties. The impact strength of glass-mat-reinforced polypropylene is approximately four to five times
that obtained with short fibers.6 The most important form of filler is E-glass, which is used typically to
reinforce thermoset polyester and epoxy resins. E-glass is boroaluminosilicate glass having low alkalimetal context and containing small percentages of calcia (CaO) and magnesia (MgO). For special
applications such as in the manufacture of aerospace materials, fibers of boron, Kevlar, PBT, and
especially carbon are generally preferred.7 Typical properties of some fiber reinforcements are shown in
Table 9.3. Table 9.4 shows the properties of some fiber-filled engineering thermoplastics composites.
The effect of mineral fillers on the properties of nylon 6/6 is shown in Table 9.5.
Table 9.3 Properties of Some Reinforcing Fibers
Glass
Property
3
Tensile strength (10 psi)
Tensile modulus (106 psi)
Density (lb/in3)
Boron
Kevlar-49
Carbon
E-Glass
S-Glass
Steel
495–500
56
0.086–0.096
525
20
0.052
400
32
0.063
500
10.5
0.092
700
12.4
0.090
600
59
0.282
The mechanical properties of particulate-filled composites are generally isotropic; that is, they are
invariant with direction provided there is a good dispersion of the fillers. On the other hand, fiber-filled
composites are typically anisotropic. In general, fibers are usually oriented either uniaxially or randomly
in a plane. In this case, the composite has maximum modulus and strength values in the direction of
fiber orientation. For uniaxially oriented fibers, Young’s modulus, measured in the orientation direction
(longitudinal modulus, EL ) is given by Equation 9.1: 7
E L = E m (1 − φ f ) + E f φ f
(9.1)
where Ef is the tensile modulus of the fiber, Em is the modulus of the matrix resin, and φf is the volume
fraction of filler.7
The performance of composites is influenced by the adhesive strength of the filler–matrix interface.
The presence of water absorbed by the filler surface and of thermal stresses generated by the differential
thermal coefficients of linear expansion between the filler and matrix materials reduces interfacial
adhesive strength. Polymers have relatively higher linear expansion coefficients than filler materials
(60 to 80 × 10–6 per °C for graphite). Therefore, in practice, interfacial adhesive strength is enhanced
by the use of coupling agents, which are usually low-molecular-weight organofunctional silanes,
stereates, or titanates that act as a bridge between the matrix resin and fiber material (Table 9.6). Coupling
Copyright 2000 by CRC Press LLC
Table 9.4 Properties of Some Engineering Thermoplastics Composites
Poly(etheretherketone)
(PEEK)
Property
Copyright 2000 by CRC Press LLC
13.2
5.65
150
—
298
Polyethersulfone
Nylon 6/6
30%
Glass
30%
Graphite
Unreinforced
30%
Glass
30%
Graphite
30% Carbon
Fibers
Unreinforced
30%
Glass
30% Carbon
Fiber
20%
Kevlar-49
20.3
31.2
15.2
24.5
30.0
30.0
11.9
25.0
36.0
18.2
11.6
22.4
12.0
25.0
25.0
4.8
13.0
27.0
8.8
3
3
3
1.4
1.7
2.5
2.0
2.3
—
—
—
1.6
1.6
2.5
2.2
3.1
572
572
4.80
60
—
392
410
410
420
40
1.2
190
485
490
490
POLYMER SCIENCE AND
Tensile strength
(103 psi)
Flexural modulus
(105 psi)
Elongation at
break (%)
Izod Impact
Strength (ft-lb/in)
Heat-deflection
temperature under
load (264 psi) (°F)
Unreinforced
Polyetherimide
233
POLYMER ADDITIVES AND REINFORCEMENTS
Table 9.5 Effect of Mineral Fillers on Nylon 6/6
Property
Specific gravity
Tensile strength (psi)
Tensile elongation (%)
Flexural modulus
(103 psi)
Izod impact(notched)
(ft-lb/in.)
Heat-deflection
temperature (°F)
Mold shrinkage
(in./in.)
Unfilled
Resin
Mica
Calcium
Carbonate
Wollastonite
Glass
Beads
Alumina
Talc
1.14
11,800
60
410
1.50
15,260
2.7
1,540
1.48
10,480
2.9
660
1.51
10,480
3.0
780
1.46
9,780
3.2
615
1.45
9,200
2.8
645
1.49
8,980
2.0
925
0.06
170
0.018
0.6
460
0.003
0.5
390
0.012
0.6
430
0.009
0.4
410
0.011
0.5
395
0.008
0.6
445
0.908
Table 9.6 Structures of Representative Coupling Agents
agents generally have two different terminal groups one of which is designed specifically to combine
with the matrix resin and the other with the fiber reinforcement. For example, the amino group in
γ-aminopropyltriethoxy silane, (C2H5O)3-Si-(CH2)3-NH2 , can react with the epoxy functionality in an
epoxy matrix, while the ethoxy groups on hydrolysis during application on the fibers form the silanol
group –Si–(OH)3 , which condenses with silanol groups on the glass fiber to form covalent Si–O–Si bonds.
The search for new reinforcements or new applications for older materials is frequently linked to the
development of new resins or new grades of the same resin to optimize performance. In broad terms,
composites are characterized by two extremes of materials. On one end are the polyester-fiberglass
automotive exterior body panels of Fiero or Corvette, representative of the relatively low cost and minimal
performance materials. Thermoplastics — including polycarbonate, polypropylene, nylon, ABS, polystyrene, high-density polyethylene, acetal copolymer, SAN, poly(butylene terephthalate), PET, thermoplastic polyurethane, and polysulfone — as well as thermosets such as cross-linkable polyesters, phenolics, and polyurethanes are the matrix resins used in these composites. On the other end of the
composite materials spectrum, are the high-cost, high-performance advanced composites typified by the
carbon- and boron-fiber-reinforced structural materials used in many aircraft and aerospace applications.
Thermosets, notably epoxies, have dominated advanced composite matrix resins. However, thermosetting
materials are being challenged because of the increasing interest in high-performance thermoplastic resin
matrices. The growing interest in thermoplastic matrix composites stems from their promise of improved
durability and toughness and possibilities for increased cost effectiveness in production and maintenance.
Thermoplastics have relatively longer shelf-life than thermosets and their reprocessability offers potential
for possible repair of design and fabricating faults. Processing of thermoplastics potentially simplifies
fabrication and reduces manufacturing cycle times, particularly by eliminating the curing cycles normally
required for cross-linking thermoset materials. This also permits access to the wider variety of manufacturing methods for processing thermoplastics. Figure 9.1 shows schematically the simplified requirements for utilizing thermoplastic resin matrices relative to thermosets in composite production.
Copyright 2000 by CRC Press LLC
234
POLYMER SCIENCE AND TECHNOLOGY
Figure 9.1 Schematic diagram illustrating the simplified requirements for utilizing thermoplastic resin matrices
compared with thermoset resin matrices.
Table 9.7 Markets and Typical Applications for Composites
Market
Aircraft/aerospace/
military
Appliances/business
equipment
Construction
Consumer products
Corrosion-resistant
equipment
Electrical/electronic
Marine
Land transportation
Typical Applications
Helicopter blades and shell; control surfaces and floor beams (Boeing 767); F-16s horizontal
stabilizer, skin, dorsal access panels and leading edge fairing, and vertical tail fin skins and louver
fin leading edge and rudder
Refrigerator compressor bases, room air conditioner parts, interior dishwasher parts, business
machine bases and housings
Reinforced plastic bathtubs and shower fixtures, dimensionally stable reinforced plastic window
lineals and doors, traffic signs, pedestrian shelters, telephone booths, and traffic kiosks
Recreational and sporting goods and recreational-vehicle equipment; microwave dinnerware and
office furniture
Underground gasoline tanks and reinforced plastic pipes and fittings; flooring for chemical
processing plants and pulp and paper mills; tank linings for the storage of crude oil; piping and
fittings for petroleum refineries; caustic and sulfur dioxide scrubbers; mixing tanks for phosphoric
acid fertilizer solutions
Fiber-optic cable; satellite dish and antenna, protective coverboards, light poles, electrical conduit,
bus bar insulation, and third-rail insulators and covers
Sailboat, powerboat, minesweeper, military craft clad with reinforced plastics
Exterior body parts, suspensions, chassis, load floors, bumpers, driveshafts, lead springs, underhood components, and automotive frames
The market for polymer composites has continued to expand in all forms of transportation (aerospace,
aircraft, marine, automotive), in the construction industry and numerous other industrial and consumer
applications. The increased use of composites in place of conventional materials is driven by their
established advantages such as corrosion resistance, high strength-to-weight ratio, and moderate costs
as well as the design flexibility offered by novel resin/reinforcement combinations together with new
processing and machinery innovations. Table 9.7 summarizes the markets and typical applications for
composites.
IV.
ALLOYS AND BLENDS
The traditional method of enhancing properties by adding fillers and reinforcements, while still effective
in some applications, has been inadequate for coping with the increasing performance requirements of
design problems and the changing material specifications. Development of new resin systems to meet
demands for high performance materials would undoubtedly take too long and would certainly be too
expensive since it would require huge investments in totally unexplored technologies and new plant
facilities. An alternative to the development of new polymers is the development of alloys and blends
that are a physical combination of two or more polymers to form a new material. The basic objective
Copyright 2000 by CRC Press LLC
POLYMER ADDITIVES AND REINFORCEMENTS
235
is to combine the best properties of each component in a single functional material that consequently
has properties beyond those available with the individual resin components and that is tailored to meet
specific requirements. Another goal is to optimize cost/performance index and improve processability
of a high-temperature or heat-sensitive polymer.
Although the terms alloys and blends are used interchangeably, they differ in the levels of inherent
thermodynamic compatibilities and in resulting properties. In general, a necessary though not sufficient
condition for thermodynamic compatibility (miscibility) is a negative change in the free energy of mixing,
∆Gm , given by Equation 9.2.
∆G m = ∆H m − T ∆Sm
(9.2)
where ∆Hm and ∆Sm are, respectively, the changes in enthalpy and entropy of mixing at temperature T.
Generally because of their large size and unfavorable energy requirements, chains from a given polymer
prefer to intertwine among themselves than with those of another polymer. Consequently, the magnitude
of ∆Sm is usually small. Therefore, for two polymers to be thermodynamically miscible, ∆Hm must be
negative, zero, or, at most, slightly positive. If ∆Hm is strongly positive, the components of a physical
mixture separate into different phases resulting in a blend. However, some polymers tend to be mutually
soluble, at least over a limited concentration range. Such are called alloys. In other words, alloys represent
the high end of the compatibility spectrum. Individual polymeric components in alloys are intimately
mixed on a molecular level through specific interactions such as donor–acceptor or hydrogen bonding
between the polymer chains of the different components.
The composition dependence of a given property, P, of a two-component polymer system may be
described by Equation 9.3: 10
p = p1 C1 + p2 C 2 + I p1 p 2
(9.3)
where P1 and P2 are the values of the property for the isolated components and C1, C2 are, respectively,
the concentrations of the components of the system. I is the interaction parameter that measures the
magnitude of synergism resulting from combining the two components. If I is positive, then the magnitude
of the property for the system exceeds that expected for a simple arithmetic averaging of the two
component properties. The system in this case is referred to as synergistic. If I is negative, then the
mixture has a property value less than that predicted from the weighted arithmetic average. This is known
as nonsynergistic. Polymer systems for which I is either zero or nearly zero are called additive blends.
They have properties that are essentially arithmetic averages of the properties of their components
(Figure 9.2).
Compatible polymer blends form single-phase systems and have a single property, like the glass
transition temperature, the value of which is generally the weighted arithmetic average of the values of
the components of the blend. At certain compositions, some compatible polymer blends exhibit strong
intermolecular attraction and hence have a high level of thermodynamic compatibility. This results in
properties superior to those of the individual components alone. Such blends display synergistic properties (e.g., tensile strength and modulus). Only a few commercially available polymers are truly
compatible. Some of these are shown in Table 9.8. Examples of the most significant commercial engineering alloys are polystyrene (PS)-modified poly(phenylene oxide) (PPO) and polystyrene (PS)-modified poly(phenylene ether) (PPE).
Incompatible polymer blends consist of a heterogeneous mixture of components and exhibit discrete
polymer phases and multiple glass transition temperatures corresponding to each of the components of
the blend. A polymer blend with completely incompatible components has limited material utility because
the components separate during processing due to lack of interfacial adhesion, which is required for
optimum and reproducible polyblend properties. Improvement in adhesion in such blends can be effected
by the addition of compatibilizers. Compatibilizing agents provide permanent miscibility or compatibility
between otherwise immiscible or partially immiscible polymers creating homogeneous materials that do
not separate into their component parts. The most effective compatibilizing agents are generally block
and graft copolymers whose polymer blocks or segments are the same as the components of the
polyblend. The inherent compatibility between the segments of the compatibilizer and each component
Copyright 2000 by CRC Press LLC
236
POLYMER SCIENCE AND TECHNOLOGY
Figure 9.2 Composition dependence of the property of a two-component polymer system: alloy properties
better than arithmetic averages; blend properties equal to or less than arithmetic averages.
Table 9.8 Representative Miscible Polymer Blends
Polymer 1
Polymer 2
Polystyrene
Poly(2,6-dimethyl-1,4-phenylene oxide)
Poly(methyl vinyl ether)
Tetramethyl BPA polycarbonates
Polycaprolactone
Poly(butadiene-co-acrylonitrile)
Chlorinated polyethylene
Poly(ethylene-co-vinyl acetate)
Poly(vinylidene fluoride)
Poly(styrene-co-acrylonitrile)
Poly(vinylidene fluoride)
Poly(vinyl chloride)
Poly(methyl methacrylate)
From Fried, J.R., Plast. Eng., 39(9), 37, 1983. With permission.
of the blend effectively ties the discrete polymer phases together. It is useful to visualize the function
of a compatibilizing agent as akin to that of the emulsifier in water–oil emulsified systems.
As we said earlier, the primary objectives in blending are to enhance material processing characteristics and optimize product performance while minimizing costs. A common strategy for achieving these
objectives is to combine a crystalline polymer with an amorphous polymer. The aim is to exploit the
strengths of each component while deemphasizing their weaknesses. Crystalline materials such as nylon,
poly(butylene terephthalate) (PBT), and poly(ethylene terephthalate) (PET) offer excellent chemical
resistance, processing ease, and stiffness, but suffer from poor impact strength and limited dimensional
stability. On the other hand, amorphous polymers such as polycarbonate (PC) and polysulfone have
outstanding impact strength and dimensional stability but poor chemical resistance. Polycarbonate–poly(ethylene terephthalate) blends combine the desirable properties of polycarbonates with the
Copyright 2000 by CRC Press LLC
237
POLYMER ADDITIVES AND REINFORCEMENTS
Table 9.9 Major Properties and Applications of Some Commercially Available Alloy/Blends
Alloy/Blends
Trade Name/
Manufacturer
Major Properties
PC/SMA
Arloy/Arco Chemical
Toughness and heat resistance,
excellent moldability
PC/ABS
Bayblend/Mobay
Improve low temperature impact
strength, processability and stiffness
SMA/ABS
Cadon/Monsanto
Improved ductility, and impact and
heat resistance
PC/ABS
Cycoloy/BorgWarner Chemicals
Improved no-load heat resistance,
impact strength
ABS/Nylon
Elemid/Borg-Warner
Chemicals
GTX/General
Electric
Mindel-A631/Union
Carbide
Chemical resistance, low-load warpage
resistance, toughness processing
High heat and chemical resistance,
dimensional stability, ductility
Platability, processability, toughness
PPE/PS
Noryl/General
Electric
Improved processing, combination of
heat resistance, excellent dimensional
stability and toughness
PC/PBT, PC/PET
Xenoy/General
Electric
Balance of chemical resistance,
toughness, low-temperature impact
strength and high temperature rigidity
PPE/Nylon
Polysulfone/ABS
Applications
Switches, power tools, food trays,
lighting fixtures, automotive
interior trim, connectors
Business machine housings and
components, industrial and
mechanical parts, switches,
terminal blocks, food trays
Automotive interior trim,
instrument panel, appliance and
equipment housings
Electroplated parts, grilles, wheel
covers, appliances, instrument
panels, telecommunications
Power tools, automotive and
agricultural components
Automotive exterior body panels,
wheel covers, mirror housings
FDA- and NSF-recognized for
food processing and food service
systems
Computer and business equipment
housings, automotive instrument
panels, interior trim, connectors,
electrical housings, medical
components
Automotive, lawn and garden
materials handling, sporting
goods, military
From Wigotsky, V., Plast. Eng., 42(7), 19, 1986. With permission.
excellent chemical resistance of PET and have been specified for use in many applications, for instance,
as replacements for metal, including automotive, lawn and garden appliances and electrical/electronic,
consumer, industrial/mechanical, sporting and recreation, and military equipment.11 A blend of PPO and
nylon is used for fenders and rocker panels of some automobiles, applications demanding chemicalresistant performance under high impact and high heat. Table 9.9 lists some alloys and blends, their
characteristic properties, and areas of application.
V. ANTIOXIDANTS AND THERMAL AND UV STABILIZERS
Polymers, during fabrication or storage or in service, may be exposed, sometimes for long periods, to
the separate or combined effects of moderate or high temperatures, ultraviolet radiation, and air or other
potential oxidants. Under these environmental conditions, polymers are susceptible to thermal, UV,
and/or oxidative degradative reactions initiated, in most cases, by the generation of free radicals. Polymer
stabilization, therefore, involves incorporation of antioxidants and thermal and UV stabilizers to minimize, if not avoid, such degradative reactions.
A. POLYMER STABILITY
Polymers deteriorate through a complex sequence of chemical reactions resulting from the separate or
combined effects of heat, oxygen, and radiation. In addition, polymers may be susceptible to attack and
mechanical failure on exposure to water (hydrolysis) or a variety of chemical agents. Molecular weight
is changed considerably in most of these reactions by chain scission and/or cross-linking. However,
deterioration can also occur without significant change in the size of the polymer molecules.
Copyright 2000 by CRC Press LLC
238
POLYMER SCIENCE AND TECHNOLOGY
1. Nonchain-Scission Reactions
Nonchain-scission reactions resulting, for example, from the application of heat, involve elimination of a
small molecule — usually a pendant group — leaving the backbone essentially unchanged (Equation 9.4).
CH2
CH
CH2
CH
R
CH2
CH
R
CH
CH
+ RH
(9.4)
R
Vinyl polymers are particularly susceptible to thermal degradation. A typical example is rigid PVC,
which is impossible to process under commercially acceptable conditions without the use of thermal
stabilizers. Unstabilized PVC undergoes dehydrochlorination near the melt processing temperature. This
involves liberation of hydrochloric acid and the formation of conjugated double bonds (polyene formation). The intense coloration of the degradation products is due to polyene formation. A second example
of a polymer that undergoes nonchain-scission reaction is poly(vinyl acetate) or PVAc. When heated at
elevated temperatures, PVAc can liberate acetic acid, which is followed by polyene formation.
2. Chain-Scission Reactions
The chemical bonds in a polymer backbone may be broken with the generation of free radicals by heat,
ionizing irradiation, mechanical stress, and chemical reactions (Equations 9.5).
H
CH2
CH
CH2
CH
R
CH2
R
C • + • CH2
CH
R
R
(9.5)
In the thermal degradation of polyethylene and polystyrene, for example, chain scission occurs through
the homolytic cleavage of weak bonds in the polymer chain. This results in a complex mixture of lowmolecular-weight degradation products. In some cases, particularly 1,1-disubstituted vinyl polymers, the
long-chain radical formed from initial chain scission undergoes depolymerization resulting in the reduction of molecular weight and formation of monomers. In a few cases, e.g., PMMA, the initial chain
scission occurs at one end of the molecule and the subsequent depolymerization results in a gradual
decrease in molecular weight. In other cases, e.g., poly(α-methylstyrene), the initial chain scission occurs
at random sites and as such there is an initial rapid reduction in molecular weight.
3. Oxidative Degradation
In the presence of oxygen or ozone, as soon as free radicals form, oxygenation of the radicals gives rise
to peroxy radicals, which through a complex series of reactions result in polymer degradation. Oxidative
degradation may occur at moderate temperature (thermal oxidation) or under the influence of ultraviolet
radiation (photooxidation). Unsaturated polyolefins are particularly susceptible to attack by oxygen or
ozone (Equation 9.6).
CH3
CH3
C
CH
C
CH2
CH2
CH3
C
Copyright 2000 by CRC Press LLC
+ H2O
O
CH3
O + HCOOH +
CH
CH2
CH2
O2, light
(9.6)
O
CH3
C
H
C
CH2
+ O3
CH2
CH
CH2
CH2
C
CH2
O
+
CH2
C
CH2
POLYMER ADDITIVES AND REINFORCEMENTS
239
4. Hydrolysis and Chemical Degradation
In addition to the separate or combined effects of heat, oxygen, and radiation, polymers may deteriorate
due to exposure to water (hydrolysis) or different types of chemical agents. Condensation polymers like
nylons, polyesters, and polycarbonates are susceptible to hydrolysis. Structural alteration of some polymers may occur as a result of exposure to different chemical environments. Most thermoplastics in
contact with organic liquids and vapors, which ordinarily may not be considered solvents for the
polymers, can undergo environmental stress cracking and crazing. This may result in a loss of lifetime
performance or mechanical stability and ultimately contribute to premature mechanical failure of the
polymer under stress.
B. POLYMER STABILIZERS
The two main classes of antioxidants are the free-radical scavengers, (primary antioxidants, radical or
chain terminators) and peroxide decomposers (secondary antioxidants or synergists). Free-radical scavengers, as the name suggests, inhibit oxidation through reaction with chain-propagating radicals, while
peroxide decomposers break down peroxides into nonradical and stable products. Commercial antioxidants include organic compounds like hindered phenols and aromatic amines, which act as free-radical
scavengers, as well as organic phosphites and thioesters that serve to suppress homolytic breakdown.
Thermal stabilizers may be based on one or a combination of the following classes of compounds:
barium/cadmium (Ba-Cd), calcium/zinc (Ca-Zn), organotin, organo-antimony, phosphite chelates, and
epoxy plasticizers. Ba/Cd stabilizer systems, which represent the largest share of the PVC stabilizer
market, are available as liquids or powders. The liquids normally employ one or more of the following
anions combined with the respective metal: nonyl phenate, octoate, benzoate, naphthenate, and neodacanoate. Conventional Ba/Cd powder stabilizer systems have one or more of the anions of the following
acids: stearic, palmitic, and lauric. The powder Ba/Cd stabilizers, which act both as thermal stabilizer
and lubricant, are normally employed in rigid and semirigid products. Ca/Zn stabilizers represent a small
volume of the stabilizer market. The best known Ca/Zn stabilizer systems are associated with FDAsanctioned products, including blown films, medical and beverage tubing, blood bags, blister packs, and
bottles. The organotin compounds are butyl-, methyl-, and octyltin and are available in liquid and solid
forms. Most organotin stabilizers are used in rigid extrusion and injection molding processes for such
products as PVC pipes, profiles, and bottles as well as fittings, siding, and clear PVC bottles.
UV radiation in the range 290 to 400 nm has potentially degradative effects on polymers since most
polymers contain chemical groups that absorb this radiation and undergo chain scission, forming free
radicals that initiate the degradative reactions. UV stabilizers are employed to impede or eliminate the
process of degradation and, as such, ensure the long-term stability of polymers, particularly during
outdoor exposure. Light stabilizers are typically UV absorbers or quenchers. The former preferentially
absorbs UV radiation more readily than the polymer, converting the energy into a harmless form.
Quenchers exchange energy with the excited polymer molecules by means of an energy transfer mechanism. Other UV stabilizers deactivate the harmful free radicals and hydroperoxides as soon as they are
formed. Pigments offer good protection for polymers by absorbing UV radiation. Carbon black, used
widely in tire manufacture, absorbs over the entire range of UV and visible radiation, transforming the
absorbed energy into less harmful infrared radiation. Pigments and carbon black cannot be used,
unfortunately, in applications where transparency is required. In these applications, stabilizers that
contribute minimal color or opacity are used. For example, transparent polymers like polycarbonate can
be protected against yellowing and embrittlement from UV light (photolysis) by incorporating compounds like benzophenone derivatives (e.g., 2-hydroxybenzophenone) and 2-hydroxybenzotriazoles.
These highly conjugated compounds are able to convert absorbed UV radiation to less harmful heat
energy without chemical change by forming a transient rearranged quinoid structure that changes back
to the initial form. Other UV stabilizers include acrylic and aryl esters, hindered amines, and metal salts.
Unlike conventional UV stabilizers, the metallic complexes interact with photoexcited polymer molecules, deactivating them by dissipating excess energy as infrared radiation.
Copyright 2000 by CRC Press LLC
240
POLYMER SCIENCE AND TECHNOLOGY
VI.
FLAME RETARDANTS13,14
Most materials, synthetic or natural, burn on exposure to temperatures that are sufficiently high. The
response of polymers to high temperatures depends on their formulation and configuration in the enduse situation. The essential goal of flame retardancy is to preserve life and prevent or at least minimize
damage to property. Therefore, the function of flame retardants in a resin formulation is ideally the
outright inhibition of ignition where possible. Where this is impossible, a flame retardant should slow
down ignition significantly and/or inhibit flame propagation as well as reduce smoke evolution and its
effects. The presence of flame retardants also tends to cause substantial changes in the processing and
ultimate behavior of commercial resins.
The burning characteristics of polymers are modified by certain compounds — including alumina
trihydrates; bromine compounds; chlorinated paraffins and cycloaliphatics; phosphorus compounds,
notably phosphate esters; and antimony oxides, which are used basically as synergists with bromine and
chlorine compounds. The halogens are most effective in the vapor phase; they act in the flame zone by
forming a blanket of halogen vapor that interferes with the propagation of the flame by interrupting the
generation of highly reactive free radicals, thus tending to quench the flame. Others such as phosphorus
or boron operate in the condensed or solid phase, minimizing the availability of fresh fuel. They form
a glaze that limits the heat and mass transfer necessary for flame propagation and/or lowers the melt
temperature of the polymer causing it to flow away from the flame. Table 9.10 shows the characteristics
and end use of various flame retardants.
Flame retardants may be classified as additives, reactives, intumescents, and nonflame-retardant
systems based on their method of incorporation in the resin formulation or their mode of action. Additive
flame retardant systems can be further classified essentially as fillers, semiplasticizers, or plasticizers
depending on their melting points and area of application. Typical additive flame retardants are halogenated additives used alone or in synergistic combinations with antimony oxide (with PS, PP, polyester,
and nylons), phosphate esters, mineral hydrates, boric acid, sodium tetraborate, and ammonium bromide.
Additive flame retardants are used with both thermoplastics and thermosets. The use of additive flame
retardants involves the addition of very high-melting inorganic materials that will reduce combustibility.
This is based on the rationale that the fewer the combustibles, the less burning will occur. Alumina
trihydrate, which dominates the flame retardant market in terms of sales volume, serves this function.
In addition, it liberates water at high temperatures and this further dampens flame energy. Alumina
trihydrate is a typical dual function filler additive (like calcium carbonate and clays), which provides
noncombustible filling plus gas or moisture release at elevated temperatures. Alumina trihydrate is used
as a primary flame-retardant additive in carpet backing, in electrical parts, and in applications employing
glass-fiber-reinforced unsaturated polyester. It is also used in sheet and bulk molding compounds for
electrical enclosures in business machine and computer housings, and in laminated circuit boards.
Alumina trihydrate offers low-cost flame retardance in epoxy systems and for wire and cable insulation,
cross-linked PE and ethylene vinyl acetate copolymers and for flexible polyurethane foams.
Flame retardants that melt or flux at or near the polymer processing temperature are known as
semiplasticizing additives, while those that truly flux during processing are referred to as plasticizing
additives. These types of flame retardants can be tailor-made for a specific polymer. Plasticizing flame
retardant filler additives provide beneficial effects in the processing of rigid polymers and quite often
improve the physical properties of the polymer. Commercial examples of plasticizing filler additives
include phosphate systems in polyphenylene oxides, phosphates or chlorinated paraffins in vinyl chloride
and vinyl acetate polymers, and octabromobiphenyl oxide and brominated aromatic compound in ABS.13
In reactive flame retardants, retardancy is built into the polymer chain through appropriate polymerization or postpolymerization techniques. For example, the current additive method of producing flame
retardant high-impact polystyrene involves blending decabromobiphenyl oxide and antimony oxide to
produce a blend with 8 to 10% combined bromine content. Reactive flame retardants with the same
bromine content can be obtained either by brominating the resin or copolymerizing styrene and brominated styrene. Reactive flame retardants include halogenated acids such as tetrabromophthalic anhydride
and tetrachlorophthalic anhydrides, halogenated or phosphorated alcohols (e.g., dibromoneopentyl glycol, dibromopropanol) and halogenated phenol bisphenol A. These materials are usually incorporated
into the polymer molecular structure through copolymerization with other reactive species.
The use of reactive flame retardants has a number of advantages over that of additive flame retardants.
With flame retardants as an integral part of the polymer chain rather than incorporated as an additive,
Copyright 2000 by CRC Press LLC
Characteristics and Applications of Flame Retardants
Market/
Characteristics
Markets
Electrical parts
Alumina Trihydrate
Resin
End Use
Resin
End Use
Unsaturated,
polyesters, epoxies,
phenolics,
thermoplastic rubbers
Switch gear, standoff
insulation, electrical
connections, junctions
boxes, circuit boards,
insulating sheet, potting
compounds
Portable power tools,
business machine
housings, computer
housings
Automotive wire,
appliance wire,
insulation and jacketing
compounds
Home laundry
equipment, air
conditioning equipment
Paneling, bathroom tubs,
shower stalls,
countertops, wall
coverings, flooring,
roofing compounds,
cushioning, mattresses
Adhesives sealants,
upholstery fabrics
Nylon 6/6, 6, PBT PET,
polycarbonate, epoxy,
polypropylene
Connectors, terminal strips,
bobbins, switches, relays, circuit
boards
Nylon 6/6, 6
polypropylene
PBT
Same as for bromine
compounds
ABS, HIPS,
polycarbonate
Business machines, computers,
TV monitors, copiers
XLPE, LDPE, EPDM,
thermoplastic rubber
Power, industrial and marine
cable; electronic wire
HIPS, ABS,
polypropylene
TV cabinetry, power tool
housings, kitchen appliances
Unsaturated polyester,
rigid polyurethanes
foam, expandable
polystyrene foam
Building panels, translucent sheet,
corrosion-resistant equipment,
thermal insulation
Unsaturated
polyester
Power tool
housings, electrical
sockets
Same as for bromine
compounds
Flexible polyurethane
foam, rigid
polyurethane foam,
unsaturated polyesters
Seating, crash padding, thermal
insulation (trucks), marine usage
(shipboard components,
coatings)
Unsaturated
polyester
Seating, panels,
sheathing
Unsaturated polyester
Wire and cable
LDPE, XLPE, PVC,
EPDM
Appliances
Unsaturated polyester
Building and
construction
Unsaturated polyester,
PVC, acrylic, EPDM,
polyurethane, flexible
polyurethane
Transportation
(automotive)
Epoxies, acrylics, PVC
Require high loading to achieve modest flame
retardance. Low cost. Low smoke advantage.
Nontoxic. Multifunctional extender. Flameretardant mechanism: endothermic (heatabsorption) cooling of flame upon release of water
of hydration
Highly efficient and versatile for wide range of applications.
Requires uses of antimony oxide. Broad range of highperformance products available, as aromatic, cycloaliphatic,
and bromine/chlorine paraffins. Vapor-phase flame-retardant
mechanism. Flame-retardant elements interrupt chemical
reactions of combustion in flame zone.
—
XLPE, LDPE,
EPDM,
thermoplastic
rubbers
Polypropylene
—
Same as for bromine
compounds
Relatively low smoke generation. One
high-performance additive and one
high-performance reactive formulation
used. Paraffins have low heat stability
and are plasticizing. Flame retardant
mechanism similar to bromine.
241
Copyright 2000 by CRC Press LLC
Chlorine Compounds
End Use
Electronic housing
and enclosures
Characteristics
Bromine Compounds
Resin
POLYMER ADDITIVES AND REINFORCEMENTS
Table 9.10
Table 9.10 (continued)
Characteristics and Applications of Flame Retardants
Phosphorus Compounds
Resin
Markets
Electrical
Antimony Oxide
End Use
Resin
End Use
Connectors, terminal strips, etc.
Nylon 6/6, 6, PBT, PET
Business machines
ABS, HIPS
Wire and cable
PVC
Communications cable
PVC, XLPE, LDPE, EPDM
Appliances
Modified PPO
Kitchen appliances, TV cabinetry
HIPS, ABS, polypropylene
PVC building wire, similar uses as for
bromine and chlorine compounds
TV cabinetry, power tools, kitchen appliances
Building and
construction
Rigid polyurethane foam, PVC,
unsaturated polyesters
Unsaturated polyester, rigid PVC,
flexible PVC
Building panels, panels, windows, insulation
covering
Transportation
(automotive)
PVC, flexible polyurethane
foam, rigid polyurethane foam
Thermal insulation, wall
coverings, flooring, sanitary
ware, laminates
Upholstery, seat cushioning,
thermal insulation (trucks)
Electronic housing
and enclosures
Characteristics
Phosphorus promotes char formation to protect substrate, and halogen
acts in vapor phase. Good thermal stability. Process with modified
PPO up to 550–600°F. Flame-retardant mechanism: condensed phase.
Flame retardant induces reactions in host resin that lead to charring
and insulation against further burning.
From Wigotsky, V., Plast. Eng., 41(2), 23, 1985. With permission.
Copyright 2000 by CRC Press LLC
Connectors, switches, circuit boards, terminal
strips
Business machines
Inert material by itself. Must be used with halogen-type compound. Used in all
polymers where halogen compounds are the selected flame-retardant system. As
a synergist, enhances efficiency.
POLYMER SCIENCE AND TECHNOLOGY
Modified polyphenylene oxide
(PPO)
Modified PPO
POLYMER ADDITIVES AND REINFORCEMENTS
243
the loss of performance of the base polymer due to diluents (other constituents) in the resin formulation
is minimized. In addition, migration and/or bloom are completely eliminated while compounding costs
are substantially reduced. Leaching of reactive flame retardants by solvents and corrosives is relatively
more difficult than leaching additive retardants.
Reactive flame retardants are used mainly with thermoset resins, particularly unsaturated polyesters,
both reinforced and unreinforced. A technique for introducing flame retardants into unsaturated polyesters
involves the use of compounds with residual unsaturation such as diallyl tetrabromophthalate, which is
used as a cross-linking agent alone or with styrene.
Intumescent flame retardance is based on the formation of a char on the surface of the resin on the
application of heat, consequently insulating the substrate from further heat and flame. Phosphorous
compounds such as inorganic or organic phosphates, nitrogen compounds such as melamine, and polyhydroxy compounds — usually pentaerythritol — are used in intumescing formulations. High loadings,
are usually required to achieve the required level of fire retardancy. This tends to degrade the physical
properties of the base polymer. Intumescent flame retardant systems are generally limited to polymers
with low processing temperature (e.g., PP) because they expand considerably on application of heat.
Nonflame-retardant systems are polymeric systems that inherently have some level of flame retardance
and therefore do not require additive or reactive flame retardants. Examples include PVC and its
compounds, poly(vinylidene chloride) films and compounds, phenolic foams, amide-imides, polysulfones, and poly(aryl sulfides).
VII.
COLORANTS15,16
As we said earlier, very few polymers are used technologically in their chemically pure form; it is
generally necessary to incorporate various additives and reinforcements to assist processing and achieve
desired properties. Unfortunately, these components also often produce a significant amount of undesirable
color and opacity in the resin. Each resin itself has its color that may vary from grade to grade or batch
to batch. For example, polystyrene crystal is transparent, whereas high-impact polystyrene has a white,
somewhat translucent, appearance while the common grade flame-retardant polystyrene is opaque. The
color of general-purpose ABS is off-white and opaque. Glass fibers, the most common reinforcements
added to nylons and polyesters, darken the color of these resins.17 The marketability of a polymer product
quite frequently depends on its color; therefore the purpose of adding a colorant to a resin is to overcome
or mask its undesirable color characteristics and enhance its aesthetic value without seriously compromising its properties and performance.
Colorants are available either as organic pigments and dyes or inorganic pigments. They may be
natural or synthetic. By convention, a dye is a colorant that is either applied by a solution process or is
soluble in the medium in which it is used, while pigments are generally insoluble in water or in the
medium of use. Dyes are generally stronger, brighter, and more transparent than pigments. As a result
of the intrinsic solubility, dyes have poor migration fastness and this restricts their use as polymer
colorants. Inorganic pigments are largely mixed metal oxides with generally good-to-excellent light
fastness and heat stability but variable chemical resistance. Organic pigments and dyes are generally
transparent and possess good brightness. The heat stability and light and migration fastness of organic
pigments range from poor to very good. Table 9.11 shows some colorants, their characteristics, and their
applications.
Colorants are used in polymers either as raw pigments (and dyes), concentrates (solid and liquid),
or precolored compounds. Precolored resins, solid and liquid concentrates, are all offsprings of the basic
dry pigments. Colorants are available in a variety of forms, including pellets, cubes, granules, powder
and liquid, and paste dispersions. Raw pigments are generally supplied as fine particles, which require
dust control measures. To optimize color development when raw pigments are used, the size of pigment
particles or agglomerates must be reduced and coated with appropriate resin. Most finished colors use
multiple pigments. This requires a homogeneous mixing of all the pigments in the formula in high-shear
mixing equipment to produce a uniform color. Precise metering into the processing machine is required
to produce consistent colors since some components of the pigment system, though present in relatively
small quantities, have strong color characteristics. Raw colorants or pigments generally cost less than
other forms of colorants, but they can be more difficult to disperse and may result in inconsistent master
batches.
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244
Table 9.11
POLYMER SCIENCE AND TECHNOLOGY
Characteristics and Applications of Some Colorants
Pigment
Characteristics
Quinacridones
(medium
red-magenta)
Perylenes (scarlet to
violet)
Diazol (scarlet to
medium red)
Azo (scarlet to bluish
reds)
Permanent red 2B
(scarlet to bluish reds)
Reds
Good to excellent heat stability in 500–525°F range,
excellent high fastness, expensive, some grades difficult to
disperse
Excellent fastness, and heat stability in 325–500°F range,
expensive, some grades difficult to disperse
Very good heat stability, light fastness, easily dispersable,
relatively expensive
Heat stability in the 450–500°F range, light fastness range
from poor to good, economical, limited light fastness in tint
Economical light red shades, good heat stability and light
fastness in mass tone and near mass tone; tendency to plate
out, limited light fastness in tints with white
Isoindolinonone
(medium orange)
Diaryl orange
(medium orange)
Oranges
Transparent, excellent heat stability and light fastness,
relatively expensive
Heat stability in 450–500°F range, fair light fastness, bright,
clear, high tinting strength, moderate price, limited light
fastness, some tendency toward bleeding
Isoindoline
(reddish yellow)
Quinophthalone
(greenish yellow)
Metal complex
(greenish yellow)
Phthalocyanine (bluish
to yellowish green)
Indanthrone
(reddish blue)
Carbazole
(reddish violet)
Iron oxides (synthetic),
red-maroon
Zinc ferrite tan
Iron oxides (natural),
siennas
Chromium oxide green
Nickel tinuium yellow
Copyright 2000 by CRC Press LLC
Yellows
Very good to excellent heat stability (500–575°F), good to
very good light fastness, transparent or opaque
Moderate light fastness, good to excellent heat stability; tint
light fastness limited
Heat stable (550–575°F), light fastness good to excellent
Greens
Heat stable to 600°F, light fast, easy to disperse
Blues
Very good light fastness and heat stable expensive
Violets
Very high tinting strength, expensive limited light fastness
Metal Oxides
Good heat stability, inexpensive, inert, poor tinting strength,
dull
Good heat stability, inert, light fast, good weatherability,
more expensive than other iron oxides
Inexpensive, color uniformity can contain impurities
Good heat stability and light fastness, excellent
weatherability and chemical resistance, inexpensive, dull,
poor tinting strength
Mixed metals oxides
Excellent weatherability, inert, easy to disperse, good
chemical resistance, poor tinting strength, weak color
Applications
Widely used in automotive
applications, propylene fibers
Extensive use in automobile,
finishes, synthetic fibers
Widely used in vinyls and
polyolefins
Polyolafin packaging, toys
housewares
Widely used in vinyls, polyolefins,
rubbers
Used for vinyls, polyolefins, and
styrenics
Used in polyolefins for packaging,
toys
Used in PVC and polypropylene
and some engineering plastics
Used in vinyls and polyolefins
Some engineering resins, PVC and
polypropylene
Very broad use in vinyls,
polyethylene styrenics,
polypropylene and some
engineering resins
Light tinting of vinyls, in rigid
PVC
Polyolefins and vinyls
Variety of plastics but not in rigid
PVC
Variety of plastics but not in rigid
PVC
Limited use in plastics,
polyethylene film bags
Can be used in most
thermoplastics and thermosets
Engineering resins, PVC siding
245
POLYMER ADDITIVES AND REINFORCEMENTS
Table 9.11 (continued)
Pigment
Inorganic browns
Cadmiums, cadmium
yellow sulfide
Chrome yellow
Characteristics and Applications of Some Colorants
Characteristics
Heat and light stable, good chemical resistance, good color
uniformity, relatively expensive
Excellent heat stability, good alkali resistance, light fast,
sensitive to moisture and acids, poor weatherability,
toxicity concerns
Bright colors, inexpensive, good hiding power, poor heat
stability, poor chemical resistance, possibly toxic
Applications
Most thermoplastics and
thermosets
Wide use in plastics including
engineering resins
Fairly broad use in thermoplastics
and thermosets
From Wigotsky, V., Plastic. Eng., 42(10), 21, 1986. With permission.
Color concentrates are intimately mixed dispersions of pigments in a base carrier resin. The pigment
content of concentrate is usually in the 2 to 30% range, but higher loadings are being developed to
enhance versatility and cost/performance benefits. The pigments and resin are subjected to high stress
during processing to promote thorough dispersion of the colorant. The concentrate is blended with the
resin material being colored in a predetermined proportion by weight known as the let-down ratio to
produce the desired color and opacity in the master batch or end product. To ensure compatibility between
the concentrate and the let-down resin, the color concentrate is generally made from the same generic
polymer as the let-down resin but with a higher melt index so as to promote ready and even mixing. A
uniform blend of concentrate must be fed either continuously by a metering device or by weight on a
batch basis into the processing equipment, which must be able to convert the blend of pellets into a
uniform melt. Color variation is produced if the melt is nonuniform or the concentrate is not completely
incorporated into the resin.
Concentrates are available in solid or liquid forms. Solid color concentrate, the major form in which
colorants are manufactured and sold, comes in pellet, cube, granulated, and powder forms. They usually
consist of dry pigments, additive components encapsulated in a base resin carrier. Dispersion and color
control are excellent, weighing is less critical, and flowability is good for easy feeding while cleanup is
easier because of the need to process color with the carrier resin.15 It is, however, necessary to ensure
that the pigment carrier and other components of the concentrate do not compromise the resistance to
heat, light, basic physical properties, and rheological compatibility of the resin. Liquid concentrates
possess many of the advantages of solid concentrates but are usually more costly. They allow lower
pigment loading and can sometimes be used at slightly lower concentrations because they cover more
surface. Liquid concentrates require less material handling and floor space for inventory, and their
production does not involve a previous heat history. Some resins, however, are unable to absorb a high
percentage of liquid.
There is currently a trend to increase the multifunctional nature of color concentrates so that color
as well as other desired properties are added to the resin system. This increases the processor’s flexibility
in meeting user’s needs and simplifies user inventory by reducing the need to stock large quantities of
tailored resins.
Precolored compounds provide the processor with a single source of color and resin. This eliminates
the need for mixing during processing and provides highly accurate color control, particularly in cases
where the base resin is subject to color variations arising from lot-to-lot color differences. In complex
part designs where resin flow may not be uniform or where equipment is unable to provide uniform
mixing, precolored compounds offer the best method of making a uniformly colored part. However,
processors sometimes tend to have preference for dry pigments and concentrates for greater flexibility
of inventory and color changeover. For example, from an inventory standpoint, it is much simpler to
stock a supply of basic natural resin to satisfy diverse, changing requirements with smaller supplies of
dry pigment or color concentrate. A processor who prefers to compound his color from pigment must,
of course, have the proper equipment and an in-depth knowledge of the process. One of the growth areas
for precolored resin is in specialized engineering applications where not only colorants but a variety of
other modifiers must be included in the compound. For example, fiber-reinforced resins that may also
contain mold-release agents, flame retardants, and other additives are best made from fully compounded
precolored resin.
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246
POLYMER SCIENCE AND TECHNOLOGY
Certain fundamental criteria must be considered in selecting a colorant for a particular application.
These include the ability of the colorant to provide the desired color effect and withstand processing
conditions and whether or not the fastness will satisfy end-use requirements. Therefore, the initial step
in the selection of a colorant is to determine whether it will provide the coloristic properties desired,
alone or in combination with others. The performance properties of a colorant are generally of two
types — those related to processing and those related to the ultimate end use. The most important
processing-related property is the heat stability of the colorant. It must be able to withstand not only
the process temperature encountered during manufacture but also, for possibly prolonged periods, the
temperature in the end-use situation.
Migration fastness is related to the solubility of the colorant in the polymer. Color migration is
manifested through bleeding, blooming, or plate-out. Bleeding is the migration from a colored polymer
film to an adjacent uncolored or differently colored material, while blooming involves colorant migration,
recrystallization, and formation of a dustlike coating on the surface of the polymer. Plate-out is characterized by the building of a coating on the metallic surfaces of processing equipment. Inadequate light
fastness of a colorant is usually manifested in the form of fading or, in the case of some colorants,
darkening. The severity and rapidity of color change depend on the chemical structure of the colorants,
its concentration in the part, and part thickness.
The compatibility of a colorant is assessed not only on the basis of the ease with which it can be
mixed with the base resin to form a homogeneous mass but also on the requirement that it neither
degrades nor is degraded by the resin. In relation to product functional properties, incompatibility of a
colorant can affect mechanical properties, flame retardancy, weatherability, chemical and ultraviolet
resistance, and heat stability of a resin through interaction of the colorant with the resin and its additives.
Flame retardancy, for example, may impinge directly on the performance of a colorant. Pressure to
produce materials with lower levels of toxic combustion products can involve organic fire retardant
additives that interact with the colorant either to negate the effect of the additives or affect the color.
VIII.
ANTISTATIC AGENTS (ANTISTATS)
Most synthetic polymers, unless specially treated, are good electrical insulators. They are therefore
capable of generating static electricity, which can be potentially costly and dangerous. Static-induced
accumulation of dust reduces the attractiveness and thus saleability of products displayed on store shelves.
The attraction of a formed part to the charged surfaces of a processing mold prevents proper ejection
of the formed part and consequently slows down production. Electrostatic charges can cause problems
when textile, films, or powders join up in automatic machinery. Sparks, and possibility explosions or
fires, can occur when static electricity is induced from plastics on nearby conductor. Damage of sensitive
semiconductors and similar complex microelectronic devices can also occur from either the direct
discharge from the conductive skin of personnel or by exposure of such devices to the close approach
of a static-charged polymer material.20
When two surfaces that are in intimate contact are rubbed together or pulled apart, static electricity
is generated. This is due to the transfer of electrons from the surface of the donor material, which
consequently becomes positively charged, to the surface of the acceptor material, which then becomes
negatively charged. For materials that are nonconductive, these static charges do not flow easily along
the surfaces and therefore remain fixed or static. Whether a material behaves as a donor or an acceptor
depends on its position in the triboelectric series (Table 9.12). For example, if nylon and propylene are
rubbed together, nylon is the donor, while polypropylene is the acceptor.
The generation of static charges is not confined strictly to nonconductors only; conductors also
generate static charges, but since they dissipate the charge quickly the level of static charges developed
by these materials is difficult to measure. When a conductor and nonconductor are separated or rubbed
against each other, the nonconductor develops a measure of static charge. The nonconductor will not
lose its charge to the ground. Consequently, the charge is removed by employing other techniques. The
use of air-ionizing bars and blowers provides an atmosphere of ionized air capable of neutralizing the
charged objects or surfaces. However, this does not provide lasting protection since it does not prevent
another charge from forming once the object is removed from the ionized air atmosphere. To ensure an
extended removal of static charges from the surface, the nonconductor must be made sufficiently
conductive to carry charge to the ground. A layer of water, even a few molecules thick, will do this
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247
POLYMER ADDITIVES AND REINFORCEMENTS
Table 9.12 The Triboelectric Series
Negative [–] end
1.
2.
3.
4.
5.
6.
7.
8.
Teflon
PVC
Polypropylene
Polyethylene
Saran
Polyurethane foam
Polystyrene foam
Acrylic
9.
10.
11.
12.
13.
14.
15.
16.
Rubber
Brass, stainless steel
Nickel, copper, silver
Acetate fiber
Steel (carbon)
Wood
Cotton
Paper
17.
18.
19.
20.
21.
22.
23.
Polyester
Aluminum
Wool
Nylon
Human hair
Glass
Acetate
Positive [+] end
satisfactorily since high conductivity is not required for the removal of static charges. Antistatic agents
(antistats) are therefore hygroscopic chemicals that can generate this layer of water by pulling moisture
from the atmosphere.
There are essentially two types of antistats that are commonly used in polymers to get rid of static
electricity: those that are applied topically and those that are incorporated internally into the polymer.
Both improve the conductance of polymer surfaces by absorbing and holding a thin, invisible layer of
moisture from the surrounding air onto the polymer surface. Topical coatings are usually applied using
wipe, spray, dip, or roller coating techniques. Topically applied antistats are particularly useful where
antistat cost control and performance are essential or when the mechanical performance of a polymer
is negatively affected by an internal additive.20 Reapplication of topical antistats may be required,
particularly in cases of high-friction end uses. Hence, antistats applied to the surface of parts are used
primarily in applications where temporary static protection is desired. The techniques for applying topical
antistats necessarily involve wasteful amounts of antistats and leave an undesirable oil-like surface.
Internal antistats are compounded directly into the polymer mix prior to processing. They then migrate
slowly and continually through the molecular interstices and the bulk polymer to its surface where they
absorb the water necessary to prevent accumulation of static charges. In this case, it is necessary to
balance the rate of such migration and the rate of surface removal of the antistat to provide long-term
protection for the part.
Major types of organic antistatic agents include quaternary ammonium compounds, amines and their
derivatives, phosphate esters, fatty acid polyglycol esters, and polyhydric alcohol derivatives such as
glycerine and sorbitol.20 Selection of the appropriate antistat depends on its compatibility with the
polymer, the end use of the part, and the desired level of antistatic activity. Other factors that need to
be considered include the effect of antistatic agent on color, transparency, and finish of the polymer part;
its possible toxicity; stability during processing; and degree of interference with physical properties and
ultimately cost effectiveness.
IX.
PROBLEMS
9.1. Explain the following observations:
a. The magnitude of an electrostatic charge increases with increasing intimacy of contact and the speed
of separation of the two materials whose surfaces are in contact with each other.
b. Graft copolymers of PVA/EVA are available commercially as flexible films. Applications include
outdoor exposed materials such as roofing, pond membranes, and swimming pool covers as well as
medical uses such as processing of blood.
c. Code specification of PVC wire and cable used in building has been raised from 60 to 90°C. This
has caused a reformulation from the use of DOP and/or DINP to phthalates such as DUP, DTDP,
and UDP, which are used in combination with trimellitate ester such as trioctyl trimellitate.
d. Although phthalate esters less volatile than DOP are used in most automotive interior applications,
trialky trimellitates are recommended.
e. Plasticizers sanctioned for food-contact applications include soya and linseed oil epoxides and various
adipates.
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POLYMER SCIENCE AND TECHNOLOGY
f. Furniture covered with tablecloths and upholstery made with plasticized PVC sometimes get discolored and tacky. Suggest a possible solution to the problem.
g. Polystyrene is completely immiscible with PVC. Styrene–maleic anhydride (SMA) copolymers, on
the other hand, show a degree of miscibility with PVC that assures part integrity and toughness.
h.
Blend
Properties
PC/PBT
PC/PET
Toughness, chemical resistance
Toughness, chemical resistance, and high temperature rigidity
i. ABS is more susceptible to oxidation than polystyrene.
j. PVC requires incorporation of thermal stabilizers and antioxidants but not flame retardants.
k. Black pigments are not normally used as colorants for rigid PVC used for home siding and window
profiles, while pigments containing heavy metals are usually avoided as colorants for polymers used
for toys and food wrapping.
9.2. The front and rear bumper beams on the 1993 Toyota Corolla were made from polypropylene/fiberglass
composite. Estimate the longitudinal modulus of these beams assuming a fiberglass (E-glass) composition of 50%.
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
Kamal, M.R., Plast. Eng., 38(11), 23, 1982.
Wigotsky, V., Plast. Eng., 40(12), 19, 1984.
Beeler, A.D., and Finney, D.C., Plasticizers, in Modern Plastics Encyclopedia, 56(10A), 212, 1979–80.
Theberge, J.E., J. Elastomers Plast., 14, 100, 1982.
Mock, J.A., Plast. Eng., 39(2), 13, 1983.
Kamal, M.R., Plast. Eng., 38(12), 31, 1982.
Fried, J.R., Plast. Eng., 39(9), 37, 1983.
Wigotsky, V., Plast. Eng., 44(10), 25, 1988.
Castagno, J.M., Plast. Eng., 42(4), 41, 1986.
Kienzle, S.Y., Plast. Eng., 43(2), 41, 1987.
Wigotsky, V., Plast. Eng., 44(11), 25, 1988.
Wigotsky, V., Plast. Eng., 42(7), 19, 1986.
Sutker, B., Plast. Eng., 39(4), 27, 1983.
Wigotsky, V., Plast. Eng., 41(2), 23, 1985.
Wigotsky, V., Plast. Eng., 40(9), 21, 1984.
Wigotsky, V., Plast. Eng., 42(10), 21, 1986.
Gordon, S.B., Plast. Eng., 39(11), 37, 1983.
Marvuglio, P., Colorants, in Modern Plastics Encyclopedia, 56(10A), 154, 1979–80.
Roe, D.M., Color concentrates, in Modern Plastics Encyclopedia, 56(10A), 162, 1979–80.
Halperin, S.A., Antistatic agents, in Modern Plastics Encyclopedia, 56(10A), 153, 1979–80.
Walp, L.E., Plast. Eng., 42(8), 41, 1986.
Copyright 2000 by CRC Press LLC
Chapter 10
Polymer Reaction Engineering
I.
INTRODUCTION
Several important differences exist between the industrial production of polymers and low-molecularweight compounds:6,7
• Generally, polymers of industrial interest have high molecular weights, usually in the range of 104 to
107. Also, in contrast to simple compounds, the molecular weight of a polymer does not have a unique
value but, rather, shows a definite distribution. The high molecular weight of polymers results in high
solution or melt viscosities. For example, in solution polymerization of styrene, the viscosity can
increase by over six orders of magnitude as the degree of conversion increases from zero to 60%.
• The formation of a large polymer molecule from small monomer results in a decrease in entropy. It
follows therefore from elementary thermodynamic considerations that the driving force in the conversion process is the negative enthalpy gradient. This means, of course, that most polymerization reactions
are exothermic. Consequently, heat removal is imperative in polymerization reactions — a problem that
is accentuated by the high medium viscosity that leads to low heat transfer coefficients in stirred reactors.
• In industrial formulations, the steady-state concentration of chain carriers in chain and ionic polymerization is usually low. These polymerization reactions are therefore highly sensitive to impurities that
could interfere with the chain carriers. Similarly, in step-growth polymerization, a high degree of
conversion is required in order to obtain a product of high molecular weight (Chapters 6 and 7). It is
therefore necessary to prevent extraneous reactions of reactants and also exclude interference of
impurities like monofunctional compounds.
• The quality of a product from a low-molecular-weight compound can be usually improved by such
processes as distillation, crystallization, etc. However, if the performance of a product from a polymerization process is inadequate, it is virtually impossible to upgrade its quality by subsequent processing.
Given these possible differences in the production processes between polymers and low-molecularweight compounds, it is vitally important to choose the most suitable reactor and operating conditions
to obtain the required polymer properties from a polymerization reaction. This demands a detailed
knowledge of the phenomena that occur in the reactor. This, in turn, requires an accurate model of the
polymerization kinetics, the mass and heat transfer characteristics of the polymerization process. Our
approach in this chapter is largely qualitative, and our treatment involves a discussion of the various
polymerization processes, followed by a brief review of polymerization reactors. It is hoped that this
approach will enable the reader to gain insight into the complex problem of selecting a reactor for a
specific polymerization reaction.
II.
POLYMERIZATION PROCESSES
Polymerization processes may be conveniently classified as homogeneous or heterogeneous. In homogeneous polymerization, as the name suggests, all the reactants, including monomers, initiators, and
solvents, are mutually soluble and compatible with the resulting polymer. On the other hand, in heterogeneous systems, the catalyst, the monomer, and the polymer product are mutually insoluble. Homogeneous polymerization comprises bulk (mass) or solution systems while heterogeneous polymerization
reactions may be categorized as bulk, solution, suspension precipitation, emulsion, gas phase, and
interfacial polymerization.
A. HOMOGENEOUS SYSTEMS
1. Bulk (Mass) Polymerization
In bulk polymerization the reaction mixture consists essentially of the monomer and, in the case of chain
growth polymerization, a soluble initiator and possibly modifiers. In the case of homogeneous bulk
0-8493-????-?/97/$0.00+$.50
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POLYMER SCIENCE AND TECHNOLOGY
Table 10.1 Heats of Polymerization
of Some Monomers
Monomer
Ethylene
Propylene
Isobutylene
Butadiene
Vinyl chloride
Vinyl acetate
Acrylic acid
Ethyl acrylate
Methyl methacrylate
Styrene
Isoprene
Vinylidene chloride
Acrylonitrile
a
Heat of Polymerization,
Btu/lba
1530–1660 (gas to solid)
860
370
620 (1,4 addition)
650
445 (at 170°F)
400 (at 166°F)
340
245
290
470
330
620
Value given for liquid monomer converted to a
condensed solid polymer at 77°F unless indicated
otherwise.
polymerization, the product polymer and monomer are miscible. Since polymerization reactions are
generally exothermic, the temperature of polymerization depends on the polymerization system. Mixing
and heat transfer become difficult as the viscosity of the reaction mass increases.
Bulk polymerization is widely practiced in the manufacture of step-growth polymers. However, since
condensation reactions are not very exothermic and since the reactants are usually of low activity, high
temperatures are required for these polymerization processes. Even though medium viscosities remain
low throughout most of the course of polymerization, high viscosities are generally experienced at later
stages of the reaction. Such high viscosities cause not only problems with the removal of volatile byproducts, but also a possible change in the kinetics of the reaction from a chemical-controlled regime
to a diffusion-controlled one. To obtain a product of an appropriate molecular weight, therefore, proper
cognizance must be given to this problem in the design of the reactor.
Organic systems have low heat capacities and thermal conductivities. Free-radical reactions are highly
exothermic in nature. This coupled with the extremely viscous reaction media for these systems prevents
effective convective (mixing) heat transfer, leading, as a consequence, to very low overall heat transfer
coefficients. The problem of heat removal is accentuated at higher conversions because the rates of polymerization and heat generation are usually enhanced at these stages of reaction. This leads to the development
of localized hot spots or runaway reactions, which if uncontrolled may be ultimately disastrous. The
occurrence of local hot spots could result in the discoloration and even possible degradation of the polymer
product, which usually has a broadened molecular weight distribution due to chain transfer to polymer.
Because of the above heat transfer problems, bulk polymerization of vinyl monomers is restricted to
those with relatively low reactivities and enthalpies of polymerization. This is exemplified by the
homogeneous bulk polymerization of methyl methacrylate and styrene (Table 10.1). Some polyurethanes
and polyesters are examples of step-reaction polymers that can be produced by homogeneous bulk
polymerizations. The products of these reactions might be a solid, as in the case with acrylic polymers;
a melt, as produced by some continuous polymerization of styrene; or a solution of polymer in monomer,
as with certain alkyd-type polyesters.
Sheets, rods, slabs, and other desired shapes of objects are produced from poly(methyl methacrylate)
in batch reactors by keeping at least one dimension of the reaction mass thin, thereby facilitating heat
transfer. Typically, the monomer containing a small amount of an initiator such as benzoyl peroxide is
poured between two glass plates separated by a flexible gasket of poly(vinyl chloride) tubing and held
together by spring clips to compensate for shrinkage. Depending upon the thickness, the filled mold is
heated from 45 to 90°C for about 12 to 24 h. After cooling, the molds are stripped from the casting and
the sheets are annealed at 140 to 150°C. The resulting sheet has good optical properties, but the process
Copyright 2000 by CRC Press LLC
POLYMER REACTION ENGINEERING
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Figure 10.1 Vertical column reactor for the continuous bulk polymerization of styrene. (From Winding, C.C.
and Hiatt, G.D., Polymeric Materials, McGraw-Hill, New York, 1961. With permission.)
suffers from a number of problems. These include possible bubbles from dissolved gases, long curing
times, and large shrinkage (about 21%). These problems may be overcome by using prepolymerized
syrup rather than the pure monomer. The syrup is prepared by carefully heating methyl methacrylate
monomer containing 0.02 to 0.1% benzoyl peroxide or azo-bis-isobutyronitrile in a well-agitated stainless
steel vessel to initiate polymerization. A good syrup is obtained after a prepolymerization cycle of 5 to
10 min at a batch temperature of 90°C.
Another way of circumventing the heat transfer problems is by continuous bulk polymerization. An
example is the polymerization of polystyrene, which is carried out in two stages. In the first stage, styrene
is polymerized at 80°C to 30 to 35% monomer conversion in a stirred reactor known as a prepolymerizer.
The resulting reaction mass — a viscous solution or syrup of polymer in monomer — subsequently
passes down a tower with increasing temperature. The increasing temperature helps to keep the viscosity
at manageable levels and also enhances conversion, which reaches at least 95% at the exit of the tower
(Figure 10.1). By removal of the heat of polymerization at the top of the tower and proper temperature
control of the finished polymer at the bottom of the tower, an optimum molecular weight may be achieved
and channeling of the polymer may be minimized.
Bulk polymerization is ideally suited for making pure polymeric products, as in the manufacture of
optical grade poly(methyl methacrylate) or impact-resistant polystyrene, because of minimal contamination of the product. However, removal of the unreacted monomer is usually necessary, and this can
be a difficult process. This may be achieved in vacuum extruders where the molten polymer is extruded
under vacuum to suck off the residual monomer.
Example 10.1: Explain why bulk polymerization is generally more suited for step-growth polymerization than for chain-growth polymerization.
Solution: The major problems in bulk polymerization are heat removal and mixing. Both step-growth
and chain-growth polymerization are exothermic. But the enthalpy of polymerization for chain-growth
reactions is of the order of 15 to 20 kcal/g mol compared with 2 to 6 kcal/g mol for step-growth
polymerization. Also, chain-growth polymerization is characterized by high viscosities even at low
conversions due to the generation of polymers early in the reaction, whereas viscosities are generally
low in step-growth polymerization until the later stages of the reaction (high conversions). Thus the
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252
POLYMER SCIENCE AND TECHNOLOGY
much higher exothermic nature of chain-growth polymerization, coupled with the difficulty of mixing
the reaction mixture due to high viscosities, leads to much lower heat transfer efficiencies in chaingrowth polymerization than in step-growth polymerization.
Example 10.2: Purified styrene monomer is charged along with initiators into an aluminum prepolymerization vessel. Polymerization is carried out at about 90°C to 30% conversion. The resulting syrup
is then poured into molds where the reaction is completed. Comment on the probable molecular weight
distribution of the product polymer.
Solution: This process is essentially batch polymerization of styrene. Most of the conversion takes place
in the finishing trays. Because of the relatively poor heat transfer in these trays, the polymerizing mass
may reach high temperatures at some spots. Consequently, the resulting polystyrene will have a broad
molecular weight distribution, with very high molecular weights being produced at low temperatures
and lower molecular weights produced at high temperature.
B. Solution Polymerization
In solution polymerization, the monomer, initiator, and resulting polymer are all soluble in the solvent.
Solution polymerization may involve a simple process in which a monomer, catalyst, and solvent are
stirred together to form a solution that reacts without the need for heating or cooling or any special
handling. On the other hand, elaborate equipment may be required. For example, a synthetic rubber
process using a coordination catalyst requires rigorous exclusion of air (to less than 10 ppm); moisture;
carbon dioxide; and other catalyst deactivators from the monomer, solvent, and any other ingredient
with which the catalyst will come in contact before the reaction. In addition, exclusion of air prevents
the tendency to form dangerous peroxides. To avoid product contamination and discoloration, materials
of construction also need to be selected with the greatest care.
Polymerization is performed in solution either batchwise or continuously. Batch reaction takes place
in a variety of ways. The batch may be mixed and held at a constant temperature while running for a
given time, or for a time dictated by tests made during the progress of the run. Alternatively, termination
is dictated by a predetermined decrease in pressures following monomer consumption. A continuous
reaction train, on the other hand, consists of a number of reactors, usually up to about ten, with the
earlier ones overflowing into the next and the later ones on level control, with transfer from one to the
next by pump.
As the reaction progresses, solution polymerization generally involves a pronounced increase in
viscosity and evolution of heat. The viscosity increase demands higher power and stronger design for
pumps and agitators. The reactor design depends largely on how the heat evolved is dissipated. Reactors
in solution polymerization service use jackets; internal or external coils; evaporative cooling with or
without compression of the vapor or simple reflux-cooling facilities, a pumped recirculation loop through
external heat exchanger; and combinations of these. A typical reactor has agitation, cooling, and heating
facilities; relief, temperature level, and pressure connections; and, frequently, cleanout connections in
addition to inlet and outlet fittings.
Solution polymerization has certain advantages over bulk, emulsion, and suspension polymerization
techniques. The catalyst is not coated by polymer so that its efficiency is sustained and removal of
catalyst residues from the polymer, when required, is simplified. Solution polymerization is one way of
reducing the heat transfer problems encountered in bulk polymerization. The solvent acts as an inert
diluent, increasing overall heat capacity without contributing to heat generation. By conducting the
polymerization at the reflux temperature of the reaction mass, the heat of polymerization can be
conveniently and efficiently removed. Furthermore, relative to bulk polymerization, mixing is facilitated
because the presence of the solvent reduces the rate of increase of reaction medium viscosity as the
reaction progresses.
Solution polymerization, however, has a number of drawbacks. The solubility of polymers is generally
limited, particularly at higher molecular weights. Lower solubility requires that vessels be larger for a
given production capacity. The use of an inert solvent not only lowers the yield per reactor volume but
also reduces the reaction rate and average chain length since these quantities are proportional to monomer
concentration. Another disadvantage of solution polymerization is the necessity of selecting an inert
solvent to eliminate the possibility of chain transfer to the solvent. The solvent frequently presents
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POLYMER REACTION ENGINEERING
253
hazards of toxicity, fire, explosion, corrosion, and odor problems not associated with the product itself.
Also, solvent handling and recovery and separation of the polymer involve additional costs, and removal
of unreacted monomer can be difficult. Complete removal of the solvent is difficult in some cases. With
certain monomers (e.g., acrylates) solution polymerization leads to a relatively low reaction rate and
low-molecular-weight polymers as compared with aqueous emulsion or suspension polymerization. The
problem of cleaning equipment and disposal of dirty solvent constitutes another disadvantage of solution
polymerization.
Solution polymerization is of limited commercial utility in free-radical polymerization but finds ready
applications when the end use of the polymer requires a solution, as in certain adhesives and coating
processes [i.e., poly(vinyl acetate) to be converted to poly(vinyl alcohol) and some acrylic ester finishes].
Solution polymerization is used widely in ionic and coordination polymerization. High-density polyethylene, polybutadiene, and butyl rubber are produced this way. Table 10.2 shows the diversity of polymers
produced by solution polymerization, while Figure 10.2 is the flow diagram for the solution polymerization of vinyl acetate.
C. HETEROGENEOUS POLYMERIZATION
1. Suspension Polymerization
Suspension polymerization generally involves the dispersion of the monomer, mainly as a liquid in small
droplets, into an agitated stabilizing medium usually consisting of water containing small amounts of
suspension and dispersion agents. The catalyst or initiator is dissolved in the monomer if the monomer
is a liquid or included in the reaction medium if a gaseous monomer is used. The ratio of monomer to
dispersing medium ranges from 10 to 40% suspension of monomer or total solids content of polymer
at the finish of polymerization. When polymerization is completed, the polymer suspension is sent to a
blowdown tank or stripper where any remaining monomer is removed, using a vacuum or antifoaming
agent if necessary. Several stripped batches are transferred and blended in a hold tank. The slurry mixture
is finally pumped to a continuous basket-type centrifuge or vibrating screen where the polymer is filtered,
washed, and dewatered. The wet product, which may still contain as much as 30% water or solvent, is
dried in a current of warm air (66 to 149°C) in a dryer. It is then placed in bulk storage or transferred
to a hopper for bagging as powdered resin or put through an extruder for forming into a granular pellet
product. Figure 10.3, which shows a flow sheet of the suspension polymerization of methyl methacrylate,
is typical of many suspension polymerization processes for the production of thermoplastic resins.
For the monomer to be dispersible in the suspension system, it must be immiscible or fairly insoluble
in the reaction medium. In some instances, partially polymerized monomers or prepolymers are used to
decrease the solubility and also increase the particle size of the monomer. The initiators employed in
the polymerization reaction are mainly of the peroxide type and, in some cases, are azo and ionic
compounds. Examples include benzoyl, diacetyl, lauroyl, and t-butyl-peroxides. Azo-bis-isobutyronitrile
(AIBN) is one of the most frequently used azo initiators, while aluminum and antimony alkyls, titanium
chloride, and chromium oxides are typical ionic initiators. The amount of catalyst used depends on the
reactivity of the monomer and the degree of polymerization, varying from 0.1 to 0.5% of the weight of
the monomer.
Apart from the monomer itself, the most important ingredient in suspension polymerization is the
suspension agent. Even though it is used in relatively small amounts (0.01 to 0.50% weight of monomer),
it is vital to the successful control of the polymerization process and the uniformity of the product
obtained. The major problem in suspension polymerization is in the formation and the maintenance of
the stability of the thermodynamically unstable droplets as they are slowly transformed from a highly
mobile immiscible liquid, through the viscous, sticky stage to the final solid beads (rigid granules)
without their coalescence or agglomeration into a conglomerate mass. During the transformation of the
liquid monomer droplet to the solid resin, the viscous or sticky phase first appears when 10 to 20%
conversion has occurred. This phase persists for up to 75 to 80% conversion before the particles take
on a nonsticky solid appearance. The tendency for agglomeration of the particles, which is particularly
critical at the stage when the particles become sticky, is prevented by proper agitation and the use of
suspending agents. The stabilizing agents are employed in two ways. (1) Surface-acting agents (surfactants) such as fatty acids and some inorganic salt such as magnesium and calcium carbonates, calcium
phosphate, titanium and aluminum oxides reduce the surface tension between water and the monomer
droplets, thus providing greater stability for this interface. They also reduce the surface viscosity of the
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254
Table 10.2
Monomer
POLYMER SCIENCE AND TECHNOLOGY
Typical Solution-Polymerization Processes
Product
Solvent
Conjugated
dienea
Synthetic rubber
b
Various
Isobutylene +
isoprene
Ethylene alone,
or with
1-butene
Butyl rubber
Methyl chloride AlCl2
Ethylene
Vinyl acetate
c
Coordination,
or alkyllithium
40
50
Atm
–140
Chrome oxide on 400–500
silica alumina
base
275–375
Peroxygenic
210–480
Polyvinyl acetate Alcohol, ester,
or aromatic
Resin
Water
Peroxygenic
compound
Latex adhesive
Water
for tire cord
Laminating resin Water NH4OH
NaOH
Acrylamide +
acrylonitrile
Acrylate
Resin
Maleic
anhydride +
styrene +
divinylbenzene
Water-soluble
thickener
Acetone or
benzene
Ammonium
persulfate
Free-radical
initiator
Benzoyl
peroxide
Ethylene +
propylene +
a diene
Epichlorohydrin
EPT rubber
Hydrocarbon
Coordination
Polyepichlorodydrin
elastomer
Thermosetting
resin
Water
Adhesive coating Ethyl acetate
Phenol +
drying oil +
hexamethylenet
etramine
Propylenee
Polypropylene
Polyoxymethylene
500–2,000
atm
Isolation
Steam coagulation,
slurrying in hot
water, extrusion
into hot water,
evaporation of
solvent on drum,
or dryer
Slurrying in hot
water
Precipitation with
water then steam
stripping
Precipitation
Ethylene glycol Variousd
Resorcinol +
formaldehyde
Melamine +
formaldehyde
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Temperature
(°F)
Linear
Cyclohexane,
polyethylene,ho pentane, or
mo- or
octane
copolymer
Polyethylene
Ethylene
Ureaformaldehyde
Bisphenol A +
Polycarbonate
phosgene
resin
Dimethyl
Polyester resin
terephthalate +
ethylene glycol
Formaldehyde
Pressure
(Psig)
Catalyst
to 104
Precipitation
320–570
Distillation of
solvent and
recovery of the
polymer as a melt
Not isolated but
used as a solution
Atm
Ambient
Atm
Spray dried, or
used as a
solution
165–175
Atm
Atm
Refluxing
temperature
Refluxing
temperature
Not isolated but
used as a solution
Neutralized with
ammonium
hydroxide and
used as a solution,
or precipitated
with petroleum
ether
Steam coagulation
200–500
100
Cyclohexane, or Organoether
aluminum
compound
Ester-alcohol
H3PO4
mixture
Autogenous
pressure
–20 to 210
Atm
350, 200, and
185 in stages
Not isolated but
used as solution
for paint vehicle
Hexane
Coordination
175
150–170
Hexane
Anionic typef
Atm
–60 to 160
Precipitates as
formed
Precipitates as
formed
POLYMER REACTION ENGINEERING
Table 10.2 (continued)
a
b
c
d
e
f
255
Typical Solution-Polymerization Processes
For example, 1,3-butadiene or isoprene
Includes hexane, heptane, an olefin, benzene, or a halogenated hydrocarbon. Must be free from moisture, oxygen, and other
catalyst deactivators.
TiCl4, an aluminium, alkyl, and cobalt halide are reported to be used to make Ameripol CB cis-polybutadiene.
In the transesterification step, inorganic salts, alkali metals or their alkoxides, or Cu, Cr, Pb, or Mn metal are used. In the
next step, the catalyst is not disclosed.
Isotactic polymers are not usually formed completely in solution but precipitate in the course of reaction.
Amines, cyclic nitrogen compounds, arisine, stibine, or phosphine.
From Back, A.L., Chem. Eng., p. 65, August 1, 1966.
Figure 10.2 Continuous process for production of low viscosity polyvinyl acetate in solution. (From Back, A.L.,
Chem. Eng., p. 65, August 1, 1966. With permission.)
Figure 10.3 Flow sheet for the suspension polymerization of methyl methacrylate. (From Church, J.M., Chem.
Eng., p. 79, August 1, 1966.)
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POLYMER SCIENCE AND TECHNOLOGY
Table 10.3 Composition and Reaction Conditions for Some Suspension Polymerization Systems
Polymer
Suspension
Mediuma
Catalyst Systema
Temperature
Pressure
Time
Cycle
Polymer
Slurry
0.1 oxygen (air), or
0.5 di-tert-butyl
peroxide
0.3 aluminum triethyl
or chromium oxide
0.2 lauroyl peroxide
350–400 F
800–1000 atm
1–2 min
10%
180–250 F
150–200 psi
1–3 h
20%
130–140 F
100–200 psi
10–12 h
18%
Polyethylene
(low density)
1,000 water
100 benzene
Polyethylene
(high density)
Polyvinyl
chloride
500 cyclohexane or
heptane
300 water
0.2 polyvinyl alcohol,
or 0.5 gelatin
0.05 emulsifier
350 water
0.5 benzoyl peroxide
0.8 polyacrylic acid
0.7 lauryl sulfonate
0.3 diacetyl peroxide
400 water
0.5 methyl cellulose
0.2 calcium phosphate
600 water
0.1 isobutylazonitrile
0.3 polyvinyl alcohol
0.1 lauryl sulfonate
Polymethylmethacrylate
Polystyrene
Polytetrafluoroethylene
a
160–180 F
—
6–8 h
23%
190–200 F
—
3–4 h
20%
80–140 F
—
2–3 h
12%
Concentrations expressed as part of materials per 100 parts of monomer.
From Church, J.M., Chem. Eng., p. 79, August 1, 1966.
droplets. (2) Water-soluble polymers like gelatin, methyl cellulose, poly(vinyl alcohol), starches, gums,
and poly(acrylic acids) and their salts raise the viscosity of the suspending aqueous medium and act as
protective coatings.
Suspension polymerization reactors are generally vertical, agitated (or stirred tank) vessels usually
made of stainless steel or glass-lined carbon steel. Reactors are provided with agitators with a paddle
or anchor-type stirrer of speed in the range 20 to 60 rpm, with baffles in some cases to enhance dispersion.
The most important design for the suspension polymerization reactor is the temperature control, which
must ensure a close degree of accuracy. Reactors must therefore be capable of removing the heat of
polymerization, which may be quite appreciable. For example, the heats of polymerization for vinyl
chloride and styrene are 650 Btu/lb and about 300 Btu/lb, respectively (Table 10.1). This heat is released
over a relatively short period (5 to 10 h). Consequently, reactors are jacketed and water cooled. The
overall heat transfer coefficient for a glass-lined steel unit is 55 to 70 Btu/h · ft2 · °F. The difference in
the heat transfer coefficients between the two reactor types is due to the additional heat transfer resistance
offered by the glass layer. Glass lining, of course, helps to reduce reactor fouling problems, but glass
thickness must be minimized so as not to compromise reactor heat transfer capability.
As the reactor size increases, problems are generally encountered with heat transfer surfaces. Even
when dimensional similarity is maintained, heat transfer area does not increase in direct proportion to
the reactor volume. For example, for a cylindrical vessel, the increase in jacket heat-transfer area for
the straight side is proportional to the volume raised to the 0.67 power. Therefore it is frequently necessary
to provide additional heat transfer surface. This is commonly achieved by using the baffles required for
agitation as cooling aids. However, extreme care must be exercised in the design of supplemental baffles
to avoid creation of dead volumes. Polymer buildup in these spots contributes to poor product properties
and creates operational problems such as the plugging of valves, lines, and pipes by chunks of polymers
from these spots. Chilled or refrigerated water is often used for heat removal in lieu of adding supplementary heat-transfer area. Hot water or low-pressure steam may also be used to heat up reactants to
initiate polymerization. Table 10.3 summarizes the typical composition and reaction conditions for a
number of suspension polymerization systems.
We indicated in our discussion of bulk polymerization that one way of reducing heat transfer problems
is to conduct the reaction in thin sections. In suspension polymerization, this concept is utilized practically
in its extreme by dispersing and suspending monomer droplets (0.0001 to 0.50 cm diameter) in an inert
nonsolvent, which is almost always water. This is achieved by maintaining an adequate degree of
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POLYMER REACTION ENGINEERING
257
turbulence and an interfacial tension between the monomer droplets and the water. This way, the monomer
droplets and the resulting viscous polymer form the discontinuous (dispersed) phase in the continuous
aqueous phase. This, combined with the added heat capacity of water, enhances the overall heat transfer
efficiency. Kinetically, therefore, each of the suspended particles has the characteristics of a miniature
bulk reactor without a mechanism for internal mixing but with a good surface-to-volume ratio for efficient
heat removal.
Suspension polymerization, also known as bead, pearl, or granular polymerization because of the
physical nature of the product polymer, has numerous attractive features. First, the use of water as the
heat exchange medium is more economical than the organic solvents used in most solution polymerizations. Second, with water as the dispersed phase acting as the heat-transfer medium, the removal of the
excessive heat of polymerization presents minimal problems, and control of temperature is relatively
simple. Another advantage is the quality of the product obtained. Separation and handling of the polymer
product are relatively easier than in emulsion and solution polymerizations. The product is far easier to
purify than emulsion systems; very little contamination occurs, with only trace amounts of catalyst,
suspension, and dispersing agents remaining in the resin. Only a minimum amount of these ingredients
are used in the polymerization process and much of these are removed in the subsequent purification steps.
Suspension polymerization is the most widely used process for making plastic resins both in terms
of the number of polymer products and in tonnage production. Practically all of the common thermoplastic resins, including some of the newer polymers, are made by this method. Styrene, methyl methacrylate, vinyl chloride, vinylidene chloride, vinyl acetate, the fluorocarbons, and some gaseous monomers, including ethylene, propylene and formaldehyde, may be polymerized by the suspension
polymerization process.
Example 10.3: Describe briefly the potential implications with respect to process operation and/or the
resulting product of the following problems.
a. Power or equipment failure in solution polymerization
b. Addition of excess amount of initiator in suspension polymerization
Solution:
a. The reaction must be stopped in an appropriate way depending on the nature of the reactants.
Failure to stop a highly exothermic polymerization during an extended power loss can be
potentially dangerous since agitators and compressors stop functioning and the necessary heat
transfer to the coolant is effectively terminated. The reaction may proceed to a violent stage or
a completely solidified mass depending on the reactants. A deactivator such as a ketone, an
alcohol, or water is effective with a coordination-type initiator. In some cases, e.g., the phenolic
type, the reaction is effectively terminated by lowering the temperature and adding more solvent.
For a free-radical polymerization reaction, a free-radical acceptor (“shot stop”) halts the polymerization. These additives are charged into the reactor where necessary or, preferably, after the
batch leaves the reactor. In the case of a train of reactors, the reactor in which the rate of
polymerization is highest is dumped into an off-grade vessel and terminated there.
b. An excess of initiator should be avoided because of the undesirable effects it has on process
control and product quality. Some of these adverse effects include low-molecular-weight polymer
formation resulting from chain termination, difficult control of temperature due to enhanced
reaction rate and exothermic heat resulting in gel formation and agglomerization, and decreased
thermal stability of the product polymer due to the presence of the excess peroxide causing
degradation of the product at processing temperatures.
Example 10.4: Explain the following observation. The use of internal cooling coils or an external
circulating loop and exchanger is generally not an appropriate means of providing supplemental heattransfer area in suspension polymerization.
Solution: Polymer fouling is generally a problem in suspension polymerization reactors. Supplemental
heat-transfer area in the form of coils is generally avoided because of the difficulty encountered in
cleaning the area between coils and the vessel wall.
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POLYMER SCIENCE AND TECHNOLOGY
The use of an external circulating loop and exchanger requires that a portion of the reaction mass is pumped
through an external heat exchanger and cooled and returned to the reactor. In suspension polymerization,
a controlled degree of agitation is imperative to prevent agglomeration and maintain the desired polymer
particle size. It is difficult to design the equipment and the recirculating loop to avoid zones of too little
agitation, where the coalescence of monomer droplets occurs. When the reaction slurry is pumped through
an external loop, the pump may impose an order-of-magnitude increase in the shear rate, thereby forcefully
agglomerating polymer particles as they pass through a sticky phase of polymerization. Also, some polymer
buildup is inevitable and an external circulating cooling loop will present cleaning problems.
2. Emulsion Polymerization
Emulsion polymerization was developed in the U.S. during World War II for the manufacture of GR-S
rubber (Government-Rubber-Styrene) or SBR (styrene–butadiene rubber) when the Japanese cut off the
supply of rubber from the East. Emulsion polymerization is now widely used commercially for the
production of a large variety of polymers. All polymers made by this process are addition polymers
rather than condensation polymers and require free-radical initiators. In general, an emulsion polymerization system would consist of the following ingredients: monomer(s), dispersing medium, emulsifying
agent, water-soluble initiator, and, possibly, a transfer agent. Water serves as the dispersing medium in
which the various components are suspended by the emulsifying agent. The water also acts as a heat
transfer medium. Monomers such as styrene, acrylates, methacrylates, vinyl chloride, butadiene, and
chloroprene used in emulsion polymerization show only a slight solubility in water.
Reactors for the emulsion polymerization process vary in size from 1000 to 4000 gal depending upon
production requirements. Reactors may be glass lined or made of stainless steel. Glass-lined reactors are
preferred for the production of acrylic polymer emulsions, while stainless steel is usually preferred for the
manufacture of poly(vinyl acetate) because it can be cleaned easily with a boiling solution of dilute caustic.
Both types of reactors have been employed in the production of butadiene–styrene copolymers and
poly(vinyl chloride). Reactors are necessarily jacketed for heat control purposes. In processes [e.g.,
poly(vinyl acetate) manufacture] where monomer, catalyst, and surfactant are added to the reactor incrementally and the available jacket heat-transfer area is not initially available for cooling, supplemental
cooling is both necessary and attractive. This involves the use of reflux cooling. Here, the heat of polymerization vaporizes unreacted monomer, and the monomer vapor is condensed in a reflux condenser and
returned to the reactor. In this case, however, the foaming characteristics of the latex must be determined
first since a stable foam carried into the reflux condenser will foul the exchanger surface. Reactors must
also be rated to withstand a minimum internal pressure ranging from 50 psi for acrylic, methacrylic and
acrylic–styrene vinyl acetate and its copolymers to at least 300 psi for vinyl chloride homopolymers and
copolymers. Figure 10.4 shows a flow sheet for a typical emulsion polymerization plant.
In order to understand the quantitative relations governing emulsion polymerization kinetics, it is
necessary to give a qualitative description of the process.
a. Distribution of Components
A typical recipe for emulsion polymerization in parts by weight consists of 180 parts of water, 100 parts
of monomer, 5 parts of fatty acid soap (emulsifying agent), and 0.5 parts of potassium persulfate (watersoluble initiator). The question, of course, is how these components are distributed within the system.
By definition, soaps are sodium or potassium salts of organic acids, for example, sodium stearate:
O
[CH3 (CH2)16
O-] Na+
C
(Str. 1)
R
When a small amount of soap is added to water, the soap ionizes and the ions move around freely. The
soap anion consists of a long oil-soluble portion (R) terminated at one end by the water-soluble portion.
(
Copyright 2000 by CRC Press LLC
O
C
O-
)
(Str. 2)
259
POLYMER REACTION ENGINEERING
Figure 10.4 Flow sheet of typical emulsion polymerization plant. (From Gellner O., Chem. Eng., p. 74, August 1,
1966.)
In other words, soap anions consist of both hydrophilic and hydrophobic groups. In water containing a
partially soluble monomer molecule, the soap anion molecules orient themselves at the water–monomer
interfaces with the hydrophilic ends facing the water, while the hydrophobic ends face the monomer
phase. Each monomer droplet therefore has a protective coat of negative charge. Consequently, the
emulsified monomer droplet is stabilized not only by the reduction of surface tension but also by the
repulsive forces between the negative charges on its surface.
Above a critical concentration of the emulsifying agent known as the critical micelle concentration
(CMC), only a small fraction of the emulsifying agent is dissolved in the water. The bulk of the emulsifier
molecules arrange themselves into colloidal particles called micelles. The micelles remain in dynamic
equilibrium with the soap molecules dissolved in water. Arguments persist with regard to the shape of the
micellar aggregate, but energy considerations favor a spherical arrangement with the hydrophilic (polar)
groups on the surface facing the aqueous phase while the hydrophobic chains are arranged somewhat
irregularly at the interior. Each micelle consists of 50 to 100 soap molecules. Proposed rodlike-shaped
micelles range in length from 1000 to 3000 Å and have diameters that are approximately twice the length
of each soap molecule. The number and size of micelles depend on the relative amounts of the emulsifier
and the monomer. Generally, large amounts of emulsifier result in larger numbers of smaller size particles.
The presence of soap or emulsifying agents considerably enhances the solubility of a water insoluble
or sparingly soluble monomer. It has been demonstrated by X-ray and light scattering measurements
that in the presence of monomers, micelles increase in size, a clear manifestation of the occupation of
the hydrophobic interior part of the micelles. Meanwhile, a very small portion of the monomer remains
dissolved. However, the bulk of the monomer is dispersed as droplets are stabilized, as discussed above,
by the emulsifier. Consequently, when a slightly water-soluble monomer is emulsified in water with the
aid of soap and agitation, three phases are present: the aqueous phase with small amounts of dissolved
soap and monomer, the emulsified monomer droplets, and the monomer-swollen micelles. The level of
agitation dictates the size of the monomer droplets, but they are generally at least 1 µm in diameter. The
emulsifier in micelle form and monomer concentrations would typically be in the range of 1018 micelles
per ml and 1010 to 1011 droplets per ml, respectively. Figure 10.5 is a schematic representation of the
components of the reaction medium at various stages of emulsion polymerization.
b. Locus and Progress of Polymerization
When the water-soluble initiator potassium persulfate is added to an emulsion polymerization system,
it undergoes thermal decomposition to form sulfate radical anion:
S2O8-
Copyright 2000 by CRC Press LLC
heat
2SO4-•
(Str. 3)
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POLYMER SCIENCE AND TECHNOLOGY
Figure 10.5 Representation of stages of an ideal emulsion polymerization. (–O) An emulsifier molecule; (M),
a monomer molecule; (P) a polymer molecule; and (R·) a free radical. (a) Prior to initiation; (b) polymerization
stage 1; shortly after initiation; (c) polymerization stage 2; all emulsifier micelles consumed; (d) polymerization
stage 3; monomer droplets disappear; and (e) end polymerization.
The water-soluble radical anions react with monomer dissolved in aqueous phase to form soap-type free
radicals:
SO4-• + (n + 1)M
50 – 60°C
SO4
(CH2 - CX2)n CH2
CX2 •
(Str. 4)
Given the three phases present in an emulsion polymerization system, the locus of polymerization can
conceivably be in the monomer droplets, in the aqueous phase within the micelles, or possibly at an
interface. Some polymerization obviously takes place in the aqueous phase but with a limited contribution
to the overall polymerization because of the low solubility of the monomer in water. The monomer
droplets also do not provide the loci for polymerization because the negatively charged sulfate anions
find the soap-stabilized monomer droplets virtually impossible to penetrate. Also, the primary sulfate
radical anions are oil insoluble. The absence of polymerization in the monomer droplets has been verified
Copyright 2000 by CRC Press LLC
POLYMER REACTION ENGINEERING
261
Figure 10.6 Stages in an emulsion polymerization. (From Rudin, A., The Elements of Polymer Science and
Engineering, Academic Press, New York, 1982. With permission.)
experimentally. Consequently, emulsion polymerization takes place almost exclusively in the micelles.
This is because of two reasons: (1) A micelle has a dimension in the range 50 to 100 Å while, as we
said earlier, the dimension of the monomer droplet is at least 1 µm (10,000 Å). Since the surface/volume
ratio of a sphere is 3/R and even though the total volume of micelles is considerably less than that of
the droplets, the micelles present a much greater total surface area. (2) The concentration of the micelles
is higher than that of the monomer droplets (1018 vs. 1011).
Emulsion polymerization is considered to take place in three stages (Figure 10.6).
• Stage I: — Polymerization is initiated in only a small number (about 0.1%) of the micelles present
initially. As polymerization proceeds, the active micelles consume the monomers within the micelle.
Monomer depletion within the micelle is replenished first from the aqueous phase and subsequently
from the monomer droplets. The active micelles grow in size with polymer formation and monomers
contained therein, resulting in a new phase: monomer–polymer particles. To preserve their stability,
these growing monomer-swollen polymer particles absorb the soap of the parent micelles. As the number
of micelles converted to monomer-swollen particles increases and as these particles grow larger than
the parent micelles, soap from the surrounding (existing micelles and emulsified monomer droplets) is
rapidly absorbed. At about 13 to 20% conversion, the soap concentration decreases to a level below
that required to form and sustain micelles (i.e., CMC). Consequently, the inactive micelles (those
without a growing polymer) become unstable and disappear. As evidence for this, the initially low
surface tension of the aqueous emulsion rises rather suddenly due to the decrease in the soap concentration, and if agitation is stopped at this stage the monomer droplets will coalesce because they are
no longer stable. This marks the completion of stage I, which is characterized by a continuous increase
in the overall rate of polymerization (Figure 10.6).
• Stage II — At the end of stage I, the locus of further polymerization is shifted exclusively to the
monomer-swollen polymer particles since the micelles (the site of generation of new polymer particles)
have all disappeared. Polymerization proceeds homogeneously in the polymer particles by the maintenance of a constant monomer concentration within the particles through the diffusion of monomers
from the monomer droplets, which, in effect, serve as monomer reservoirs. As the polymer particles
increase in size, the size of the monomer droplets disappears. Since there is no new particle nucleation
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POLYMER SCIENCE AND TECHNOLOGY
Table 10.4 Characteristics of Various Stages of Emulsion Polymerization
Stage
Prior to initiation
Stage I
(12–20% conversion)
Stage II
(25–50% conversion)
Stage III
(50–80% conversion)
Characteristics
Dispersing medium, usually water, containing small amount of dissolved soap (emulsifier) and
monomer.
Monomer droplets each of size ca. 10,000 Å separate due to stabilization by a coat of emulsifier
molecules whose hydrophilic ends face the aqueous phase; concentration of monomer droplets
1010–1011 per ml.
Beyond CMC, emulsifier molecules number 50–100 form spherically shaped micelles each of
size 40–50 Å; some micelles are swollen by monomer and have dimensions 50–100 Å; micelle
concentration is about 1018 per ml.
Low surface tension due to emulsifier concentration.
Polymerization initiated in ca. 0.1% of initially present micelles.
As monomers in active micelles are consumed, and then replenished by diffusion of monomers
from the aqueous phase and subsequently from monomer droplets, the micelles form swollen
particles that are stabilized by their absorbing soap molecules from neighboring inactive micelles
and emulsified monomer droplets.
End of stage marked by disappearance of inactive micelles and increase in surface tension;
agitation needed to prevent coalescence of monomer droplets.
Low concentration of dissolved monomer molecules.
No dissolved emulsifier or emulsifier micelles.
Polymerization occurs exclusively in monomer-swollen polymer (latex) particles through
diffusion of monomers from monomer droplets.
Polymer particles grow while monomer droplets decrease in size.
No new particle nucleation (i.e., number of latex particles is constant) and since monomer
concentration within particles is constant, rate of polymerization is constant.
End of stage marked by disappearance of monomer droplets.
No dissolved monomer, dissolved emulsifier, emulsifier micelles, monomer droplets or monomerswollen micelles.
Since monomer droplets have disappeared, a supply of monomers from the monomer reservoir
(i.e., monomer droplets) is exhausted, hence rate of polymerization drops with depletion of
monomers in latex particles.
At end of polymerization (i.e., 100% conversion) system contains polymer particles, i.e.,
400–800 Å dispersed in aqueous phase.
at this stage, the number of polymer particles remains constant. As a result of this and the constant
monomer concentration within the particles, this stage is characterized by a constant rate of reaction.
• Stage III — At an advanced stage of the polymerization (50 to 80% conversion), the supply of excess
monomer becomes exhausted due to the disappearance of the monomer droplets. The polymer particles
contain all the unreacted monomers. As the concentration of monomer in the polymer particles
decreases, the rate of polymerization decreases steadily and deviates from linearity. The characteristics
of various stages of emulsion polymerization are summarized in Table 10.4.
c. Kinetics of Emulsion Polymerization
A number of questions need to be resolved from the qualitative description of emulsion polymerization
given in the previous section. For example, it is necessary to consider whether the diffusion of monomers
to the polymer particles is high enough to sustain polymerization given the low solubility of monomer
in the aqueous phase. It is also important to know the average radical concentration in a polymer particle.
Also, the validity of the assumption that only the monomer–polymer particles capture the radicals
generated by the initiator needs to be established convincingly. The answers to these questions were
provided by Smith and Ewart3 and this forms the basis for the quantitative treatment of the steady-state
portion of emulsion polymerization.
(1) Rate of Emulsion Polymerization
In emulsion polymerization, the rate of generation of free radicals is about 1013/m-s while the number
of monomer–polymer particles for typical recipes, N, is in the range 1013 to 1015 particles/ml of the
aqueous phase. Consequently, if all the initiator radicals are captured by the monomer–polymer particles,
each particle will acquire, at the most, a radical every 1 to 100 s. It can be shown that if a particle
contains two radicals, mutual annihilation of radical activity will occur within a time span of the order
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POLYMER REACTION ENGINEERING
of 10–13 s. This is much smaller than the interval between radical entry into the polymer particle. It is,
therefore, perfectly safe to assume that the entry of a second radical with an active polymer particle
results in immediate bimolecular termination. The particle remains dormant until another radical enters
about 1 to 100 s later to reactivate polymerization. The activity of the particle will once again be
terminated after 1 to 100 s due to the entry of another radical and the cycle is repeated. It follows,
therefore, that at any given point in time a particle will either have one or zero radicals. This, in turn,
means that a given particle will be active half of the time and dormant the other half of the time. By
extension of this argument, at any given instant of time, one half of the polymer particles will contain
a single radical and be active while the other half will remain dormant. The polymerization rate is given by:
R p = k p [M] [M ⋅]⋅
Therefore, the polymerization rate per cubic centimeter of water is given by
Rp = kp
N
[M]
2
(10.1)
where N = number of polymer particles per cubic centimeter of aqueous phase
kp = homogeneous propagation rate constant
[M] = monomer concentration in the polymer particles
Notice that Equation 10.1 predicts a direct dependence of the polymerization rate on the number of
particles but not on the rate of radical generation. The equation holds true, of course, only when the
radicals are being produced. Again, since the derivation of the equation is pivoted on the argument that
each particle does not contain more than one radical at a given time, it follows that the particle size is
not large enough to violate this condition.
Figure 10.7 is an experimental verification of the linear dependence of the rate of polymerization on
the number of particles and monomer concentration. The polymerization rate increases with an increase
in soap concentration due to the increase in N with soap concentration.
Example 10.5: The rate of diffusion, I, into a sphere of radius r is given by
I = D4πr∆C
where D = diffusion coefficient (cm2/s)
∆C = concentration difference between the surface of the sphere and the surroundings
In the emulsion polymerization of styrene at 60°C the diffusion coefficient is 10–10 cm2 s–1 and the
termination rate constant is 3 × 107 l mol–1 s–1. Show that the rate of termination of the initiating radicals
in the aqueous phase is of the order 103 radicals/ml/s. What is the average lifetime of a radical in the
aqueous phase? Assume that the concentration of the radicals at the surface of the polymer particles is
zero.
Solution: The rate of termination Rt is given by
R t = 2 k t [M ⋅]
2
where [M·] = the overall concentration of radicals.
Since the concentration of radicals within and at the surface of the polymer particles is zero then
∆C = [M·] = the concentration of radicals in the surrounding aqueous phase
[M·] = I/D4πr
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POLYMER SCIENCE AND TECHNOLOGY
Figure 10.7 Polymerization of isoprene in emulsion at 50°C using 0.3 g of K2S2O8 per 100 g of monomer, and
with the amounts of soap (potassium chlorate) indicated in weight percent and in molality m. (From Harkins,
H.B., J. Am. Chem. Soc., 69, 1428, 1947. With permission.)
For emulsion polymerization, the rate of generation of radicals from the initiator is of the order of 1013
radicals/cm3/s; the number of polymer particles is about 1014 particles/cm3; and the average diameter of
polymer particles is 1000 Å. The rate of diffusion of radicals to the polymer surface is
I=
rate of generation of radicals
concentration of polymer particles
radicals 1
cm3
−1
−1
= 1013
= 10 radicals s
14
3
cm s 10 particle
r=
[M ⋅] =
1
× 1000 Å = 5 × 10 −6 cm
2
(10
−5
10 −1 radicals s−1
cm 2 s−1 (4 π) 5 × 10 −6 cm
)
(
)
= 108 radicals cm −3
k t = 3 × 10 7
1
1000 cm3
1 mol
l
= 3 × 10 7
l
mol − s
6.02 × 10 23 molecules s
= 5 × 10 −14 cm 3 molecules−1 s−1
(
)[
R t = 2 5 × 10 −14 cm 3 molecules−1 s−1 108 radicals cm3
= 103 radicals cm −3 s−1
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]
2
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POLYMER REACTION ENGINEERING
Average lifetime of a radical from generation to its capture by a polymer particle is
concentration of radicals
rate of radical generation
=
108 radicals cm −3
1013 radicals cm −3 s−1
= 10 −5 s
d. Degree of Polymerization
Whenever a primary radical enters an inactive polymer particle, polymerization occurs as it would in
normal homogeneous polymerization. In this case the rate of polymerization is given by
rp = k p [M]
(10.2)
where kp is the propagation rate constant. The rate of capture of primary radicals is given by
ri
N
rc =
(10.3)
where ri is the rate of generation of primary radicals (in radicals per milliliter per second). From our
discussion in the previous section, the growth of a polymer is terminated immediately following the
entry of another radical. Therefore the rate of termination should be essentially equal to the rate of
capture of primary radicals. The degree of polymerization, in the absence of transfer, should then be the
ratio of the rate of polymer growth to the rate of capture of primary radicals.
Xn =
Xn =
rp
rc
=
kp [M]
ri N
(10.4)
kp N [M]
ri
Both the degree of polymerization and the rate of polymerization show a direct variation with the number
of polymer particles N. However, unlike the rate of polymerization, the degree of polymerization varies
indirectly with the rate of generation of primary radicals. This is as should be expected intuitively since
the greater the rate of radical generation, the greater the frequency of alternation between polymer
particle growth and dormancy and therefore the lower the chain length.
e. The Number of Particles
Equations 10.1 and 10.4 show that the number of polymer particles is crucial in determining both the
rate and degree of polymerization. The mechanism of polymer particle formation indicates clearly that
the number of polymer particles will depend on the emulsifier, its initial concentration (which determines
the number of micelles), and the rate of generation of primary radicals. Smith and Ewart3 have shown that
r
N = k i
µ
0.4
(a [ E])
s
0.6
(10.5)
where k is a constant with a value between 0.4 and 0.53; µ s is the rate of increase in volume of a polymer
particle, as is the interfacial area occupied by an emulsifier molecule; and [E] is the soap or emulsifier
concentration. Note that all the units are in cgs. Equations 10.1, 10.4, and 10.5 establish the quantitative
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POLYMER SCIENCE AND TECHNOLOGY
relations between the rate and degree of polymerization and the rate of generation of primary radicals
and the emulsifier concentration.
In bulk polymerization, the rate of polymerization depends directly on the rate of initiation. However,
the degree of polymerization is inversely related to the rate of initiation. Consequently, an increase in
the rate of initiation results in a high rate of polymerization but a decrease in the degree of polymerization.
This constitutes a major differences between bulk and emulsion polymerization. In emulsion polymerization, it is possible to increase the rate of polymerization by increasing the concentration of polymer
radicals through a high initial emulsifier concentration. If the rate of initiation (generation of primary
radicals) is kept constant, the degree of polymerization is increased as well.
f. Deviations from Smith–Ewart Kinetics
The Smith–Ewart kinetic theory of emulsion polymerization is simple and provides a rational and
accurate description of the polymerization process for monomers such as styrene, butadiene, and isoprene,
which have very limited solubility in water (less than 0.1%). However, there are a number of exceptions.
For example, as we indicated earlier, large particles (> 0.1 to 0.5 cm diameter) may and can contain
more than one growing chain simultaneously for appreciable lengths of time. Some initiation in, followed
by polymer precipitation from the aqueous phase may occur for monomers with appreciable water
solubility (1 to 10%), such as vinyl chloride. The characteristic dependence of polymerization rate on
emulsifier concentration and hence N may be altered quantitatively by the absorption of emulsifier by
these particles. Polymerization may actually be taking place near the outer surface of a growing particle
due to chain transfer to the emulsifier.
Emulsion polymerization has a number of unique advantages compared with other polymerization
methods. The viscosity of the reaction mass is relatively much less than that of a comparable true solution
of polymers in the same molecular weight range. This, coupled with the increased heat capacity due to
the presence of water, results in excellent heat transfer and creates a physical state that is much easier
to control. Efficient removal of heat of polymerization is one of the factors determining the rate at which
polymer may be produced on a commercial scale: efficient heat transfer permits faster rates to be used
without overheating the mass and thus avoiding possible polymer degradation. As indicated above, it is
possible to obtain both high rates of polymerization and relatively high-molecular-weight polymers
through high emulsifier concentration and low initiator concentration. In bulk, solution, or suspension
polymerization, rapid polymerization rates can be attained only at the expense of lower-molecular-weight
polymers, except in anionic polymerization. In contrast to suspension polymerization, where there is a
high risk of agglomeration of polymer particles into an intractable mass, emulsion polymerization is
suitable for producing very soft and tacky polymers. Relatively low viscosity with high polymeric solids
is advantageous in many applications. The latex product from emulsion polymerization can be used
either directly or through master-batching to obtain uniform compounds that find useful applications in
coatings, finishes, floor polishes, and paints. Emulsion polymerization, however, has some drawbacks.
The large surface area presented by the tiny surfaces of a large number of small particles is ideal for
absorption of impurities, thus making the product polymer impure. For example, the presence of watersoluble surface-active agents used in the polymerization process results in some degree of water sensitivity of the polymer itself, while ionic materials such as surfactants and inorganic salts result in poor
electrical properties of the final polymer. Only free-radical-type initiators can be used in emulsion
polymerization. This precludes the possibility of producing stereoregular polymers by this method.
Example 10.6: From the data given below for the emulsion polymerization styrene in water at 60°C:
a. Calculate the rate of polymerization.
b. Show that the number average degree of polymerization Xn is 3.52 × 103
c. Estimate the number of polymer chains in each. Data:
kp = 176 l mol–1 s–1
ri = 5 × 1012 radicals cc–1 s–1
N = 1013 particles cc–1
[M] = 10 M
Latex particle size = 0.10 µm
Particle density = 1.2 g/cc
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POLYMER REACTION ENGINEERING
Solution: a.
Rp = kp ⋅
kp =
N
[M]
2
176 l
1000 cc 1
cc
5
= 176
= 1.76 × 10
mol-s
mol − s
l mol − s
N = 1013
1 mol
particles
particle
= 1013
cm3 6.023 × 10 23 particles
cm3
= 1.66 × 10 −11
[M] = 10 M =
mol
cm3
10 mol
mol 1 l
−2
−1
= 10
= 10 mol cc
l
l 1000 cc
1.76
cm3
mol
−11 mol
10 −2
× 10 5
Rp =
1.66 × 10
3
mol − s
cm
cm3
2
= 1.46 × 10 −8 mol cc −1 s−1
b.
Xn = k p N
[M]
ri
ri = 5 × 10 −12
1 mol
radicals
cm3 s 6.023 × 10 23 radicals
= 8.30 × 10 −12
mol
cm3− s
cm3
mol 1012 cm3− s
−11 mol
10 −2 3
Xn = 1.76 × 10 5
1.66 × 10
3
mol − s
cm
cm 8.30 mol
= 3.52
52 × 103
c.
Volume of a particle =
4 3
π r , r = 0.05 µm = 0.05 × 10 −4 cm
3
(
= 4.19 r 3 = 4.19 × 5 × 10 −6 cm
)
3
= 5.24 × 10 −16 cm 3
Density of each particle = 1.2 g cc
(
)(
Mass of each particle = 1.2 g cm 3 5.24 × 10 −16 cm 3
= 6.29 × 10 −16 g
Molecular wt of styrene = 104
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)
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POLYMER SCIENCE AND TECHNOLOGY
Each particle contains
1 g mol
6.29 × 10 −16 g
monomer units
104 g
= 6.05 × 10 −18 g mol monomer units
= (6.05) × 10 −18 g-mol monomer units)
monomer
23
6.023 × 10
g-mol monomer
= 3.64 × 10 6 monomers
Since Xn = 3.5 × 103 or 3.52 × 103 monomer per chain
10 −3 chains
Chains per particle = 3.64 × 10 6 monomers
3.25 monomer
(
)
= 103 × 103 chains
3. Precipitation Polymerization
Precipitation polymerization, also known as slurry polymerization, involves solution systems in which
the monomer is soluble but the polymer is not. It is probably the most important process for the
coordination polymerization of olefins. The process involves, essentially, a catalyst preparation step and
polymerization at pressures usually less than 50 atm and low temperatures (less than 100°C). The resultant
polymer, which is precipitated as fine flocs, forms a slurry consisting of about 20% polymer suspended
in the liquid hydrocarbon employed as solvent. The polymer is recovered by stripping off the solvent,
washing off the catalyst, and if necessary, extracting any undesirable polymer components. Finally, the
polymer is compounded with additives and stabilizers and then granulated.
The suspension of the polymer flocs in the solvent produces a physical system of low viscosity that
is easy to stir. However, problems may arise due to settling of the polymer and the formation of deposits
on the stirrer and reactor walls. Most industrial transition-metal catalysts are insoluble, and consequently
polymerization occurs in a multiphase system and may be controlled by mass transfer. Therefore, the
type of catalyst employed exerts a larger influence on parameters and reactor geometry.
4. Interfacial and Solution Polycondensations
Monomers that are very reactive are capable of reacting rapidly at low temperatures to yield polymers
that are of higher molecular weight than would be produced in normal bulk polycondensations. The best
and most widely used reactants are organic diacid chlorides and compounds containing active hydrogens
(Table 10.5):
O
C
O
Cl + -NH2
base
C
NH
+
HCl
(Str. 5)
In interfacial polymerization a pair of immiscible liquids is employed, one of which is usually water
while the other is a hydrocarbon or chlorinated hydrocarbon such as hexane, xylene, or carbon tetrachloride. The aqueous phase contains the diamine, diol, or other active hydrogen compound and the acid
receptor or base (e.g., NaOH). The organic phase, on the other hand, contains the acid chloride. As the
name suggests, this type of polymerization occurs interfacially between the two liquids. In contrast to
high-temperature polycondensation reactions, these reactions are irreversible because there are no significant reactions between the polymer product and the low-molecular-weight by-product at the low
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269
POLYMER REACTION ENGINEERING
Table 10.5 Typical Interfacial/Solution Polycondensation Reactions
Active
Hydrogen Compound
Acid Halide
Product
O
-NH2
O
H
C
N
Polyamide
C
Cl
O
-NH2
H
O
H
N
C
N
H
O
N
C
Polyurea
C
Cl
O
-NH2
Polyurethane
Cl
C
O
O
O
O
-OH
Polyester
Cl
C
O
O
C
O
-OH
Polycarbonate
Cl
C
Cl
O
C
O
From Williams, D.J., Polymer Science and Engineering, Prentice-Hall, Englewood Cliffs, NJ,
1971. With permission.
temperatures employed. Consequently, the molecular weight distribution is a function of the kinetics of
the polymerization system; it is not determined statistically as in normal equilibrium polycondensations.
The rate of reaction is controlled by the rate of monomer diffusion to the interface. This obviates the
necessity to start the reaction with stoichiometric quantities of reactants. Since the reactions are irreversible, high conversions are not necessarily required to obtain high-molecular-weight polymers.
In solution polycondensation, all the reactants are dissolved in a simple, inert solvent. However, for
some solution polymerizations, the solvent can facilitate the reaction. For example, a tertiary amine such
as pyridine is an acid acceptor in the solution phosgenation in polycarbonate manufacture.
CH3
n HO
C
CH3
O
OH + nCl
C
Cl
N
CH2Cl2 solution
O
CH3
C
O
O
C
CH3
(Str. 6)
+ 2nHCl
Interfacial and solution polycondensations are commercially important. For example, an unstirred interfacial polycondensation reaction is utilized in the production of polyamide fibers. Another important
application of interfacial polycondensation is the enhancement of shrink resistance of wool. The wool
is immersed first in a solution containing one of the reactants and subsequently in another solution
containing the other reactant. The polymer resulting from the interfacial reaction coats the wool and
improves its surface properties.
III.
POLYMERIZATION REACTORS
The course of a polymerization reaction and hence the properties of the resultant polymer are determined
by the nature of the polymerization reaction and the characteristics of the reactor employed in the
reaction. In other words, the reactor is essentially the heart of any polymerization process. The reactor
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POLYMER SCIENCE AND TECHNOLOGY
Table 10.6 Characteristics, Advantages and Disadvantages of Various Polymerization Types
Polymerization
Process
Characteristics
Advantages
Bulk
Reaction mixture consists
essentially of monomer;
and initiator in the case
of chain reaction
polymerization
Monomer acts as solvent
for polymers
Products relatively pure due to
minimum contamination
Enhanced yield per reactor volume
Solution
Solvent miscible with
monomer, dissolves
polymer
Heat transfer efficiency greatly
enhanced resulting in better process
control
Resulting polymer solution may be
directly usable
Suspension
Monomer and polymer
insoluble in water,
initiator soluble in
monomer
Heat removal and temperature control
relatively easier
Polymer obtained in a form that is
convenient and easily handled
Resulting polymer suspension or
granules may be directly usable
Emulsion
Monomer and polymer
insoluble in water,
initiator soluble in water
Emulsifier needed for
stabilization of system
component particularly
at initial stages of
polymerization
Polymer insoluble in
monomer or monomer
miscible with precipitant
for polymer
Physical state of the system enhances
heat transfer efficiency
Possible to obtain high rates of
polymerization and high average
chain lengths
Narrow molecular weight distribution
Latex (emulsion) often directly usable
Polymerization occurs at
interface of two
immiscible solvents,
usually water and an
organic solvent
Polymerization is rapid and occurs at
low temperatures
High conversions are not necessarily
required to obtain high molecular
weight
Unnecessary to start with
stoichiometric quantifiers of
reactants
Precipitation
Interfacial
Physical state of system permits easy
agitation
Relatively low temperatures
employed
Disadvantages
Exothermic nature of polymerization
reactions (particularly chain reaction
polymerizations) makes temperature
control of system difficult
Product has broad molecular weight
distribution
Removals of tracers of unreacted
monomer difficult
Necessary to select an inert solvent to
avoid possible transfer to solvent
Lower yield per rector volume
Reduction of reaction rate and average
chain length
Not particularly suitable for production
of dry or relatively pure polymer due
to difficulty of complete solvent
removal
Need to maintain stability of droplets
requires continuous and a minimum
level of agitation
Possibility of polymer contamination
by absorption of stabilizer on particle
surface
Continuous operation of system
difficult
Difficult to get pure polymer due to
contamination from other
components of polymerization
system
Difficult and expensive if solid
polymer product is required
Presence of water lowers yield per
reactor volume
Separation of product difficult and
expensive
Catalyst systems are special and need
careful preparation
Molecular weight distribution depends
on type of catalyst
Limited to highly reactive systems
Need appropriate choice of solvent to
dissolve reactants
affects the conversion of the monomer to the polymer. The reactor also effectively establishes the ultimate
properties of the polymer such as polymer structure, molecular weight, molecular weight distribution,
and copolymer composition. To perform its functions satisfactorily, the reactor must remove the heat of
polymerization, provide the necessary residence time, provide good temperature control and reactant
homogeneity, control the degree of back-mixing in a continuous polymerization, and provide surface
exposure. In addition, the reactor must be applicable to mass production and economical to operate.
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POLYMER REACTION ENGINEERING
Therefore, the control of polymer properties requires a careful selection of a reactor appropriate for the
particular polymerization process.
We dealt with various polymerization processes in the previous sections. We now consider polymerization reactors. Our treatment of this subject is essentially qualitative; the principal focus is to highlight
salient features of each reactor type.
Reactors may be divided into three simple, idealized model categories: batch reactor, tubular or plug
flow reactor, and the continuous stirred tank reactor (CSTR).
A. BATCH REACTORS
In the case of the batch reactor, the reactants are charged into the reactor and mixed properly for the
duration of the reaction and then the product is discharged. The batch reactor has essentially the following
characteristics:
• It is simple and does not need extensive supporting equipment.
• It is ideal for small-scale operations.
• The operation is an unsteady-state operation, with composition varying with time.
Now let us discuss how these features affect the various polymerization reactions and the resultant
polymer. We start by considering the general material balance equation for the batch reactor:
Rate of
Rate of
Rate of monomer loss
Rate of monomer
monomer flow = monomer flow +
due to reaction
+
accumulation
into the reactor
out of the reactor
within the reactor
within the reactor
(10.6)
For a batch reactor, the first two terms of Equation 10.6 are equal to zero since by definition nothing
flows in or out of the reactor. Consequently, the equation reduces to
Rate of monomer loss
Rate of accumulation
due to reaction
=
monomer
within the reactor
within the reactor
−
∫
t
dM
= Rp
dt
dt = −
o
∫[
[ M ] dM
Mo ]
Rp
(10.7)
(10.8)
(10.9)
For free radical polymerization,
fk
Rp = kp d
kt
12
[ I ] 1 2 [ M ]⋅
If f is independent of monomer concentration and the initiator concentration remains constant, then the
above operation is first order in monomer concentration and may be rewritten:
R p = k [M]
fk
where k = kp d
kt
[I]1/2. On substitution of Equation 10.10, Equation 10.9 becomes
∫ dt = −∫
t
o
Copyright 2000 by CRC Press LLC
(10.10)
12
[ M ] dM
[ M ]o
kM
(10.11)
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POLYMER SCIENCE AND TECHNOLOGY
Integrating this equation yields
ln
[M]
[M ]
= − kt
(10.12)
o
or
[M] = [M] o e − kt
Percent Conversion = 100
(10.13)
[M] o − [M]
= 100 (1 − e − kt )
[M] o
(10.14)
The derivation of Equation 10.13 assumes that the composition of the reaction mixture is uniform
throughout the reactor at any instant of time and the initiator concentration is constant. Good mixing is
an important way to ensure not only uniform concentration but also uniform temperature and prevents
the occurrence of localized inhomogeneities. However, the mixing efficiency for bulk polymerizations
in batch reactors, while difficult to generalize, usually varies with conversion because the viscosity and
density of the reaction mass changes continuously with reaction time. For example, the viscosity of
styrene at 150°C increases by an order of three at 60% conversion. The corresponding changes in
emulsion and suspension polymerizations are less drastic. An important assumption made in the deviation
of Equation 10.10 is that the initiator concentration remains constant. This assumption may not be
realistic under industrial conditions beyond a few percentage points of conversion. A more accurate
expression is obtained by assuming a first-order decay of the initiator:
[I] = [I o ] e − k t
(10.15)
d
Example 10.7: Calculate the time required for 10% polymerization of pure styrene at 60°C with benzoyl
peroxide as the initiator in a batch reactor. Assume that the initiator concentration remains constant. Data:
f=1
k2p/k t = 0.95 × 10–3 l/mol-s
[I] = 4.0 × 10–3 mol/l
kd = 1.92 × 10–6 s–1
Solution: Since the initiator remains constant, Equation 10.12 is applicable:
ln
[M]
[M] o
= − kt
fk
k = kp d
kt
12
[ I ]1 2
or
k2 =
k p2
kt
( f k ) [I]
d
l
−6 1
−3 mol
= 0.95 × 10 −3
1.92 × 10 4.0 × 10
mol − s
l
s
= 7.30 × 10 −12 s−2
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273
POLYMER REACTION ENGINEERING
k = 2.70 × 10 −6 s
ln
[M]
[M ]
= −2.70 × 10 −6 t
o
[ M ] − [ M ] = 0.10
[M]
10, i.e.,
M]
M
[
[ ]
o
o
= 0.9
o
ln 0.9 = −2.70 × 10 −6 t
t = 8.10 h
In chain growth reactions, temperature control is a major problem in bulk polymerization and, to a
lesser extent, in solution polymerization in batch reactors. This is due to the large increase in viscosity
of the reaction medium with conversion. The heat transfer to a jacket in a vessel varies approximately
inversely with the one-third power of the viscosity. For example, in a stirred batch (tank) reactor with
thick walls and unfavorable surface-to-volume ratio, polymerization essentially proceeds adiabatically.
A variety of methods are employed for heat removal, including heat transfer to a jacket, the internal
cooling loop, and, in the case of a vaporizable constituent, an overhead condenser through reflux. While
reactors are almost always jacketed, the use of additional heat removal devices is generally necessary
as the size of the reactor increases since the heat transfer area of the reactor increases with reactor
volume to the two-thirds power, while the rate of heat generation varies directly with the reactor volume.
As we said earlier, the use of external cooling devices, of course, depends on the polymerization. Where
the viscosity is relatively low and/or the latex stable, a portion of the reaction mixture is recycled through
a heat exchanger. This is impossible in suspension polymerization where a continuous and minimum
level of agitation is required to ensure the stability of the particles and avoid the formation of coagulate
and wall deposits in the dead volumes. Also, the high viscosity in bulk polymerization precludes the
use of external heat exchangers because the poor agitation would lead to wall deposits and a rapid loss
of cooling efficiency. The use of internal cooling coils is restricted to low viscosity reactions to avoid
poor product quality resulting from improper mixing. The idealized model batch reactor together with
its residence time is shown in Figure 10.8a.
Figure 10.8 Idealized reactors and their associated residence time distribution. (From Gerrens, H., Chem.
Technol., p. 380, June 1982.)
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274
POLYMER SCIENCE AND TECHNOLOGY
B. TUBULAR (PLUG FLOW) REACTOR
The tubular or plug flow reactor, as the name suggests, is essentially a tube or unstirred vessel with very
high length/diameter (l/d) ratio. Fluid flow in the ideal plug flow reactor is orderly, with no element of
fluid overtaking any other element. The description of a tubular reactor as plug flow connotes that each
element moves through the tube as a plug with no axial diffusion along the flow path and no difference
in the velocity of any two elements flowing through the reactor (i.e., no lateral or back-mixing). For the
ideal plug flow reactor, therefore, the residence time for all elements flowing through the tube is the same.
In principle, the tubular reactor has a favorable surface-to-volume ratio and relatively thin walls —
features that should enhance heat-transfer efficiency. In addition, like the batch reactor, it is suited for
achieving high conversions. However, tubular reactors have limited application in polymer production.
The high viscosities characteristic of polymerization reaction media present temperature control problems. In addition, the material temperature increases from the tube wall to its radius. This broad
distribution of temperature leads to a broad molecular weight distribution — a situation that is accentuated
in chain-growth polymerizations by the decrease in initiator and monomer concentrations with increasing
conversions. Since the reactor walls are cooler that the center of the reactor, there is a tendency to form
a slow-moving polymer layer on the walls. This reduces the production capacity and aggravates the heattransfer problems. A typical idealized-model tubular reactor together with the resistance time distribution
and concentration profile for a first-order reaction is shown in Figure 10.8b.
Example 10.8: The production of high-pressure low-density polyethylene is carried out in tubular
reactors of typical dimensions 2.5 cm diameter and 1 km long at 250°C and 2500 atm. The conversion
per pass is 30% and the flow rate is 40,000 kg/h. Assuming that the polymerization reaction is first order
in ethylene concentration, estimate the value of the polymerization rate constant.
Solution: Consider a mass balance about an elemental volume of the reactor.
On substitution:
vM o dp = R p dV
∫
V
o
dV
=
vMo
Now,
p=
∫
o
∫
o
dp
Rp
Mo − M
Mo
dp = −
V
p
dM
Mo
dV
=
vMo
∫
M
V
=
v
∫
M
−
dM
Mo Rp
−
dM
Rp
Mo
Mo
R p = k [M]
k
Copyright 2000 by CRC Press LLC
V
=−
v
∫
M
V
=−
v
∫
M
Mo
Mo
dM
kM
dM
or kτ = M
∫
M
Mo
dM
M
275
POLYMER REACTION ENGINEERING
--- = mean residence time.
where τ = V
v
ln
M
= − kτ
Mo
or
M = M o e − kτ
1 M
k = − ln
τ Mo
For a conversion of 30%
Mo − M
= 0.3
Mo
or
M
= 0.7
Mo
k=
1
ln 0.7
τ
2
2.5
2
V = π R2L = π
× 10 −2 × 1.0 = π (1.25) × 10 − 4
2
= 4.9 × 10 −4 m 3
Assume, for simplicity, under the conditions of polymerization ethylene behaves as an ideal gas
pv = nRT
or
v=
nRT
P
n = 40, 000 kg h =
40, 000
kg mol h
28
= 1.43 × 103 kg mol h
R = 0.08206 (l)(atm) (g mol) (K)
m3
3
= 0.08206 l × 10 −3
(atm) kg mol × 10 (K)
l
(
)
( )
= 0.08206 m 3 (atm) ( kg mol) (K)
T = 273 + 250K = 523 K
v = 1.43 × 103
= 24.55 m 3 h
Copyright 2000 by CRC Press LLC
( )
m3 (atm)
kg mol
1
.
0
08206
(523 K )
h
2500 atm
(kg mol) ( K )
276
POLYMER SCIENCE AND TECHNOLOGY
Assuming the flow rate is constant along the entire length of the reactor, then
τ=
V 4.91 × 10 −4 m3
=
= 2.0 × 10 −5 h
24.55 m3 h
v
k=−
1
1
ln 0.7 = 2.0 × 10 −5 h ln 0.7
τ
τ
k = 1.78 × 10 4 h −1
= 4.95 s−1
C. CONTINUOUS STIRRED TANK REACTOR (CSTR)
The ideal continuous stirred tank reactor is a reactor with well-stirred and back-mixed contents. As a
result, instant blending of the feed with the reactor contents is assumed to occur. The composition of
the contents of the reactor is uniform throughout the reactor. Consequently, the exit stream from the
reactor has the same composition and temperature as the reactor contents.
During polymerization with a CSTR, the monomer and the other components of the polymerization
recipe are fed continuously into the reactor while the polymerization product mixture is continually
withdrawn from the reactor. The application of the CSTR in suitable polymerization processes reduces,
to some extent, the heat removal problems encountered in batch and tubular reactors due to the cooling
effect from the addition of cold feed and the removal of the heat of reaction with the effluent. Even
though the supporting equipment requirements may be relatively substantial, continuous stirred tank
reactors are economically attractive for industrial production and consistent product quality.
In a CSTR, each element of monomer feed has an equal chance of being withdrawn from the reactor
at any instant regardless of the time it has been in the reactor. Therefore, in a CSTR, unlike in batch
and tubular reactors, the residence time is variable. The contents of a well-stirred tank reactor show an
exponential distribution of residence times of the type shown in Equation 10.15.
R( t ) = e − t τ
(10.16)
where R(t) is the residence time distribution and t is the mean residence time, which, as we said
previously, is the ratio of the reactor volume to the volumetric flow rate. The residence time distribution
influences the degree of mixing of the reaction mixture. This, in turn, determines the uniformity of the
composition and temperature of the reactor contents. As a consequence, the polymer product properties
are influenced by the residence time distribution. Figure 10.8c shows an ideal CSTR with its residence
time distribution and concentration profile. Tables 10.7 and 10.8 summarize polymerization reactions
and processes, industrially employed reactors, and resulting polymer products.
Table 10.7 Polymerization Processes and Industrially
Employed Reactors
Polymerization
Reaction
Chain
Ionic
Step growth
Copyright 2000 by CRC Press LLC
Polymerization
Process
Reactor
Batch
Bulk
Solution
Suspension
Emulsion
Precipitation
Solution
Precipitation
Solution
Interfacial
X
X
X
X
X
X
X
X
X
Plug Flow
CSTR
X
X
X
X
X
X
X
X
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POLYMER REACTION ENGINEERING
Table 10.8 Polymerization Reactions and Processes and Some Polymer Products
Reactor
Batch
Polymerization
Reaction
Free radical
Polymerization Process
Solution
Precipitation
Suspension
(bead)
Emulsion
Ionic
Solution
Condensation
Precipitation
Solution
Interfacial
Plug Flow
CSTR
Free radical
Solution
Ionic
Precipitation
Condensation
Solution
Free radical
Solution
Precipitation
Emulsions
Precipitation
Copyright 2000 by CRC Press LLC
Example
Paint resins by polymerization and often also
copolymerization of acrylates and
methacrylates
Vinlylacetate in methanol with subsequent
alcoholysis to poly(vinyl alcohol)
Polyacrylonitrile spinning solution in
dimethylformamide
Acrylonitrile in water
Azeotropic copolymerization of styrene and
acrylonitrile in methanol
Poly(vinylchloride)
Expanded polystyrene
Poly(methyl methacrylate)
Poly(vinyl acetate)
Water soluble paints
Poly(vinyl acetate)
Butadiene
Isoprene
Ethylene or propylene and their copolymers
Block copolymers such as butadiene-styrene or
styrene-butadiene-styrene
Polyether polyols block copolymers prepared
from reaction of ethylene oxide or propylene
oxide with polyhydric alcohols used for
polyurethane preparation
Polymerization of ε-caprolactam
Polyethylene-propylene copolymer
Formaldehyde resin (UF, MF, PF)
Nylon 6,6
Polyurethanes
Heat resistant aromatic polyamides and
polyimides
High pressure polymerization of ethylene to
give LDPE
Polymerization of isobutene in liquid ethylene
with BF3 catalyst
Polymerization of trioxane with BF3 etherate
as catalyst to produce polyoxymethylene
Continuous manufacture of polyurethene foam
blocks
Production of nylon 6,6 on the extruder reactor
as the last stage
Vinyl acetate esters of acrylic acid acrylonitrile
Acrylonitrile
Polymerization of vinyl chloride
Polyacrylates and polymethacrylates
Butadiene, isopreal and then copolymers
Cationic copolymerization of isobuteneisoprene with slurry AlCl3 as the initiator and
methyl chloride as diluent
AlCl3 slurry polymerization of propylene in the
presence of transition methyl catalyst and
excess monomer as a diluent
Fluidized bed reactor is used in the gas phase
polymerization; a powdered polymer is
produced in a gaseous monomer-low pressure
polymerization of ethylene (HDPE) and
propylene
278
POLYMER SCIENCE AND TECHNOLOGY
Example 10.9: Explain the molecular weight distribution shown in the following table.
Polymerization Reaction
Reactor
Batch or plug flow
Continuous stirred tank reactor
Chain-Growth
Step-Reaction
Wider than Schulz-Flory distribution
Schulz-Flory distribution
Schulz-Flory distribution
Much wider Schulz-Flory distribution
Solution: In radical chain reactions, the overall rate of polymerization, Rp , and the number-average
degree of polymerization, Xn , are functions of the initiator concentration [I], the monomer concentration
[M], and also the temperature via the temperature dependence of the individual rate constants. At constant
[M] and [I], the Schulz–Flory MWD is produced. However, if [M] and [I] vary with time, a number of
Schulz–Flory distributions overlap and thus a broader MWD is produced. In the ideal CSTR [M] and
[I] are constant and the temperature is relatively uniform. Consequently, chain polymerizations in CSTR
produce the narrowest possible MWD. In the batch reactor, [M] and [I] vary with time (decrease with
conversion) while in the tubular reactor [M] and [I] vary with position in the reactor and the temperature
increases with tube radius. These variations cause a shift in Xn with conversion and consequently a
broadening of MWD.
The molecular weight distribution also depends on the relative lifetime of the growing macromolecule
and the mean residence time. In step-growth polymerization, even though monomers disappear very
early in the reaction, high conversions are required to generate high Xn . In general, therefore, the lifetime
of a growing macromolecule is much higher than the residence time in step-growth polymerizations.
For the CSTR, the composition of the product is not altered by increasing the duration of the polymerization process. In addition, because the residence time in CSTR is variable, there is an equal chance
of finding both small and large molecules in the polymerization product. Consequently, there is a
relatively broad molecular weight distribution. On the other hand, for step-growth polymerization in
batch and tubular reactors, narrow MWD with increasing Xn is generated with increasing conversion.
IV.
PROBLEMS
10.1. Polyesters and polyamides are prepared according to the following equilibrium reactions.
O
-OH + -COOH
-NH2 + -COOH
Ke
Ka
C
O
H
O
N
C
+ H2O
+ H2O
(Str. 7)
(Str. 8)
Ke and Ka are 10 and 400, respectively.
In the manufacture of linear polyesters, the final stages of polymerization are carried out at pressures
of about 1 mm Hg and temperatures of 280°C. On the other hand, the reaction for the manufacture of
nylon 6,6 is completed at 240 to 280°C and atmospheric pressure. Explain.
10.2. Repeat Example 10.7 in this chapter but assume that the initiator concentrations show a first-order decay.
10.3. Calculate the time required for 80% conversion of pure styrene at 60°C with benzoyl peroxide as the
initiator in a batch reactor. Assume that the initiator concentration remains constant. Use the data given
in Example 10.7 in this chapter and comment on your result.
Copyright 2000 by CRC Press LLC
POLYMER REACTION ENGINEERING
279
10.4. It has been proposed that to avoid possible health problems, the polystyrene used for drinking cups
must contain less than 1% monomer. Polystyrene was prepared at 100°C by the thermal polymerization
of 10 M styrene in toluene in a batch reactor. The shift operator stopped the reaction after 2.5 h. Is the
product from this operation suitable for producing drinking cups without further purification?
Data:
k p2
kt
= 8.5 × 10 −3 l mol-s
ki = 4.2 × 10 −11 l mol − s
10.5. Estimate the adibatic increase in temperature for the batch polymerization of butadiene.
Data:
∆Hp = –18.2 kal/g mol
Heat capacity = 29.5 cal/g mol °C
10.6. For the bulk polymerization of methyl methacrylate at 77°C with azo-bis-isobutyronitrile as the initiator,
the initial rate of polymerization was 1.94 × 10–4 mol/l-s. The concentrations of the monomer and initiator
were 9 mol/l and 2.35 × 10–4 mol/l, respectively. To reduce the heat transfer problems, the polymerization
was repeated in a solution of benzene by the addition of 4.5 l of benzene with an initiator concentration
of 2.11 × 10–4 l/mol. Assuming that the rate constants are the same for both the bulk and solution
polymerizations, calculate the rate of solution polymerization of methyl methacrylate. If there is no
transfer to the solvent and the initial rate of initiation is 4 × 10–4 mol/l-s, what is the ratio of the numberaverage degree of polymerization for bulk-to-solution polymerization? Comment on your results.
10.7. It has been found that the polymer from a certain toxic monomer has excellent mechanical properties
as a food wrap. To ensure maximum conversion and obtain a relatively pure product, the monomer was
bulk polymerized in a batch reactor. Do you agree with the decision to use this polymer as a food wrap?
10.8. A typical emulsion polymerization recipe consists of 180 g water, 100 g monomer, 5 g soap, and 0.5 g
potassium persulfate. Estimate the ratio of the total surface area of micelles to that of monomer droplets.
Assume that the relative volume of micelle to a droplet is equal to the ratio of their volumes in the
polymerization recipe. The density of a micelle is 0.2 g/cc and that of a droplet is 0.8 g/cc.
10.9. The following data were found for the emulsion polymerization of vinyl acetate at 60°C:
N = 12.04 × 1014 particles/ml
Ri = 1.1 × 1012 radicals/ml-s
kp = 550 l/mol-s
[M] = 5 M
Polymer particle density = 1.25 g/cm3
a. Assuming that the Smith–Ewart kinetics is valid for emulsion polymerization up to 80% conversion,
how much time was required to obtain this conversion?
b. What is the average size of the latex polymer particles if, on the average, each particle contains 133
chains?
10.10. A batch reactor of length 2.5 m and diameter 5.046 m is filled with vinyl acetate monomer. Polymerization is carried out isothermally at 50°C. The reactor is jacketed for heat removal. What is the
temperature of the coolant?
Data:
Rate of polymerization = 10–4 mol/l-s
∆Hp = 21.3 kcal/mol
Overall heat-transfer coefficient = 0.0135 cal/cm2 s °K
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280
POLYMER SCIENCE AND TECHNOLOGY
10.11. In an emulsion polymerization of styrene in a 50 m3 CSTR, the feed contains 5.845 mol of styrene
per liter. For an average conversion of 75% in the reactor and assuming the Smith–Ewart behavior is
followed, estimate:
a. The mean residence time
b. The volumetric flow rate
Data:
kp = 2200 l/mol-s
N = 1.80 × 1014 particles/cm3
10.12. The bulk polymerization of a polymer in a plug flow reactor had been found to follow first-order
kinetics. The flow rate is 3.14 × 10–4 m3 s–1, while the reaction rate constant and residence time are
10–2 s–1 and 102 s, respectively. Calculate:
a. The conversion
b. The length of the reactor if its diameter is 2 × 10–2 m
10.13. The bulk polymerization of the poly(ethylene terephthalate) or polyisobutylene is to be carried out.
For which of these polymerizations would it be more favorable to use a plug flow reactor?
10.14. Explain why a continuous stirred tank reactor (CSTR) is not normally used in industry in the production
of nylon 6,6. What modification is needed to make (CSTR) suitable for the preparation of nylon 6,6?
10.15. The specific heats of polymerization of styrene and butene monomers are 160 and 300 kcal/kg,
respectively. For which of these monomers would solution polymerization be more appropriate?
10.16. In certain cis-butadiene processes, the volume of the diluting solvent is sufficiently high that cooling
is achieved by feeding cold diluent solvent to the reactor. For an end-use application, the final polymer
concentration was 10%. The specific heat of the solvent is 0.96 Btu/lb°F. If the final reaction temperature is 122°F, at what temperature should the solvent be fed into the reactor so as to remove all of
the heat of polymerization?
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
Winding, C.C. and Hiatt, G.D., Polymeric Materials, McGraw-Hill, New York, 1961.
Roe, C.P., Ind. Eng. Chem., 69(9), 20, 1968.
Smith, W.V. and Ewart, R.H., J. Chem. Phys., 16, 592, 1948.
Harkins, H.B., J. Am. Chem. Soc., 69, 1428, 1947.
Williams, D.J., Polymer Science and Engineering, Prentice-Hall, Englewood Cliffs, NJ, 1971.
Gerrens, H., Chem. Technol., p. 380, June 1982.
Gerrens, H., Chem. Technol., p. 434, July 1982.
Rudin, A., The Elements of Polymer Science and Engineering, Academic Press, New York, 1982.
Flory, P.J., Principles of Polymer Chemistry, Cornell University Press, Ithaca, NY, 1953.
Levenspiel, O., Chemical Research Engineer, John Wiley & Sons, New York, 1962.
Cameron, J.B., Lundeen, A.J., and McCulley, J.H., Jr., Hydrocarbon Process., B9, 50, 1980.
Diedrich, B., Appl. Polym. Symp., 26, 1, 1975.
Wallis, J.P.A., Ritter, R.A., and Andre, H., AIChE J., 21, 691, 1975.
Wohl, M.H., Chem. Eng., p. 60, August, 1966.
Back, A.L., Chem. Eng., p. 65, August 1, 1966.
Church, J.M., Chem. Eng., p. 79, August 1, 1966.
Gellner, O., Chem. Eng., p. 74, August 1, 1966.
Schlegel, W.F., Chem. Eng., p. 88, March 20, 1972.
Copyright 2000 by CRC Press LLC
Chapter 11
Unit Operations in Polymer Processing
I.
INTRODUCTION
Polymer processing may be divided into two broad areas. The first is the processing of the polymer into
some form such as pellets or powder. The second type describes the process of converting polymeric
materials into useful articles of desired shapes. Our discussion here is restricted to the second method
of polymer processing. The choice of a polymer material for a particular application is often difficult
given the large number of polymer families and even larger number of individual polymers within each
family. However, with a more accurate and complete specification of end-use requirements and material
properties the choice becomes relatively easier. The problem is then generally reduced to the selection
of a material with all the essential properties in addition to desirable properties and low unit cost. But
then there is usually more than one processing technique for producing a desired item from polymeric
materials or, indeed, a given polymer. For example, hollow plastic articles like bottles or toys can be
fabricated from a number of materials by blow molding, thermoforming, and rotational molding. The
choice of a particular processing technique is determined by part design, choice of material, production
requirements, and, ultimately, cost–performance considerations.
The number of polymer processing techniques increases with each passing year as newer methods are
invented and older ones modified. This chapter is limited to the most common polymer processing unit
operations, but only extrusion and injection molding, the two predominant polymer processing methods,
are treated in fairly great detail. Our discussion is restricted to general process descriptions only, with
emphasis on the relation between process operating conditions and final product quality. Table 11.1
summarizes some polymer processing operations, their characteristics, and typical applications.
II.
EXTRUSION1
Extrusion is a processing technique for converting thermoplastic materials in powdered or granular form
into a continuous uniform melt, which is shaped into items of uniform cross-sectional area by forcing
it through a die. As shown in Table 11.1, extrusion end products include pipes for water, gas, drains,
and vents; tubing for garden hose, automobiles, control cable housings, soda straws; profiles for construction, automobile, and appliance industries; film for packaging; insulated wire for homes, automobiles, appliances, telephones and electric power distribution; filaments for brush bristles, rope and twine,
fishing line, tennis rackets; parisons for blow molding. Extrusion is perhaps the most important plastics
processing method today.
A simplified sketch of the extrusion line is shown in Figure 11.1. It consists of an extruder into which
is poured the polymer as granules or pellets and where it is melted and pumped through the die of
desired shape. The molten polymer then enters a sizing and cooling trough or rolls where the correct
size and shape are developed. From the trough, the product enters the motor-driven, rubber-covered rolls
(puller), which essentially pull the molten resin from the die through the sizer into the cutter or coiler
where final product handling takes place.
A. THE EXTRUDER
Figure 11.2 is a schematic representation of the various parts of an extruder. It consists essentially of
the barrel, which runs from the hopper (through which the polymer is fed into the barrel at the rear) to
the die at the front end of the extruder. The screw, which is the moving part of the extruder is designed
to pick up, mix, compress, and move the polymer as it changes from solid granules to a viscous melt.
The screw turns in the barrel with power supplied by a motor operating through a gear reducer.
The heart of the extruder is the rotating screw (Figure 11.3). The thread of an extruder screw is called
a flight, and the axial distance from the edge of one flight to the corresponding edge on the next flight
is called the pitch. The pitch is a measure of the coarseness of the thread and is related to the helix
0-8493-????-?/97/$0.00+$.50
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POLYMER SCIENCE AND TECHNOLOGY
Table 11.1 Some Unit Operations in Polymer Processing and Typical Products
Process
Characteristics
Resin Employed
Typical Products
Extrusion
A process for making
indeterminate length
of thermoplastics with
constant cross-section
Most thermoplastics including
PE, PP, PVC, ABS, PS
Injection
molding
Versatile process, most
suitable for high
speed, low cost
molding of intricate
plastic parts required
in high volume
Process used for
making bottles and
other hollow plastic
parts having relatively
thin walls
Virtually all thermoplastics and
some thermoset; most
common are commodity
plastics such as PVC, PE, PP,
and PS; others include ABS,
nylon, cellulosics, acrylics
Several thermoplastics with PE
(particularly ND, PE) having
the largest volume; others
include PVC, PP, PS, ABS,
acrylics, nylons, acrylonitrile,
acetates, and PC
PE, (highest volume) PP, PVC
together account for almost all
plastics used; others include a
number of engineering
thermoplastics, including
ABS, acetal copolymers, nylon
(6 and 11), polycarbonate
Almost all thermoplastics but
most commonly used include
ABS, PP, PS, PVC polyesters;
others include acrylics,
polycarbonate, cellulosic,
nitrile resins
Pipes and tubing used for soda straws, garden
hose, drains and vents, control cable
housings, gas and water pipes; profiles, e.g.,
auto trim, home siding, storm windows;
sheets used for window glazing, refrigerator
liners, signs, plates, lighting; films for bags,
coverings, laminates, packaging; fibers or
filaments for brush bristles, rope, fishing
line; insulated wire for homes, automobiles,
appliances, telephone and electric power
distribution; coated paper for milk cartons,
meat packaging
Automobile parts, appliance housings,
camera cases, knobs, gears, grilles, fan
blades, bowls, spoons, lenses, flowers,
wastebaskets and garbage cans
Blow molding
Rotational
molding
Economical process for
the production of
hollow seamless parts
with heavy walls
and/or complex
shapes
Thermoforming
Process for forming
moderately complex
shapes that are not
readily amenable to
injection molding
Compression
and transfer
molding
Most widely used
techniques for
molding thermosets
Phenolic (largest volume), urea,
melamines, epoxy, rubber,
diallyl phthalate, alkyds
Casting
Process for converting
liquid resins into rigid
objects of desired
shape
Polyesters, nylons,
polyurethanes, silicones,
epoxies, phenolics, acrylics
Copyright 2000 by CRC Press LLC
Bottles, watering cans, hollow toys, gas tanks
for automobiles and trucks
Hollow balls, squeeze toys, storage and feed
tanks, automobile dashboards, door liners
and gearshift covers, industrial storage
tanks and shipping containers, whirlpool
tubs, recreational boats, canoes and camper
tops, hobby horses, heater ducts, auto
armrest skins, athletic balls, portable toilets
Automobile, airline and mass transportation
industries for such uses as auto headliners,
fender walls, overhead panels, aircraft
canopies; construction industry for exterior
and interior paneling, bathtubs, shower
stalls; outdoor signs; appliances, e.g.,
refrigerator liners, freezer panels;
packaging trays for meat packing, egg
cartons, fast-food disposables and
carryouts, blister packages, suitcases, tote
boxes, cups, and containers
Pot handles, electrical connectors, radio
cases, television cabinets, bottle closures,
buttons, dinnerware, knobs, handles,
replacement for metal parts in electrical,
automotive, aircraft industries
Tooling and metal-forming industries’ cast
epoxy dies used to produce airplane and
missile skins, automobile panels and truck
parts; epoxies used by artists and architects
for outdoor sculpture, churches, homes and
commercial buildings, and encapsulation in
electronic industry; cast acrylic sheets used
in airplanes, helicopters, schools
UNIT OPERATIONS IN POLYMER PROCESSING
283
Figure 11.1 Sketch of an extrusion line. (From Richards, P.N., Introduction to Extrusion, Society of Plastics
Engineers, CT, 1974. With permission.)
Figure 11.2 Parts of an extruder. (From Richards, P.N., Introduction to Extrusion, Society of Plastics Engineers,
CT, 1974. With permission.)
Figure 11.3 Parts of an extruder screw. (From Richards, P.N., Introduction to Extrusion, Society of Plastics
Engineers, CT, 1974. With permission.)
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POLYMER SCIENCE AND TECHNOLOGY
angle. The polymer is melted and pumped in the open section between the flights called the channel.
The bottom of the channel is called the root of the screw. The distance between the root and the top of
the flight is referred to as the channel depth. The hub and shank, located at the rear of the screw, fit into
the driving mechanism. The hub acts as a seal preventing the polymer material from leaking into the
machinery.
The most common and versatile extruder currently in use is the single screw extruder. Extruders are
normally characterized by the bore diameters D and the length of the barrel specified as the length-todiameter ratio, L/D. Diameters range from about 1 in. for laboratory to about 8 in. for specialized
production machines. Typical L/D ratios for commercial extruders range from 20/1 to 34/1. In general,
the shorter machines (L/D = 20/1 range) are used for elastomer processing while the longer ones are
employed for processing thermoplastics. Screws with pitch equal to diameter are referred to as squarepitched screws and have a helix angle of 17.7°. The number of turns of the flight in a square-pitched
screw gives the L/D ratio.
The channel depth changes along the screw length; it is deepest at the section under the hopper and
shallowest toward the tip. In a few cases where the channel depth changes gradually from one end to
the other, the screw is called a constant taper screw. In most cases, however, the screw is divided into
three distinct zones each with a different channel depth. These zones are referred to as the feed, transition,
and metering sections based on the differing functions. The feed section, 1 to 10 diameters long, picks
up the resin under the hopper and advances it into the externally heated barrel to begin melting. The
compression section or the transition zone, about 5 diameters long, compacts the loosely packed polymer
feed and in the process eliminates air pockets. It melts and forms the resin into a continuous stream of
molten material. The frictional force generated between the resin, the barrel wall, and the rotating screw
(viscous energy dissipation) is an important energy source for melting the resin. The metering or pump
section ensures a uniform flow rate and generates the pressure needed to force the polymer melt through
the rest of the extruder and out through the die. Screws are also sometimes described in terms of
compression ratio, which is essentially the ratio of the channel depth of the first turn at the rear end of
the screw to that of the channel at the last turn. Screws vary from a compression ratio of two for a lowcompression screw to four for a high-compression screw.
Various modifications of the single screw design are available. Multiscrew extruders are also in current
use for specialized application for which the single screw designs are inefficient. Examples of such
applications include production of large-diameter PVC pipe and processing of heat-sensitive materials
or resins that must leave the die at relatively low temperatures. Screw configuration for multiscrew
extruders may involve intermeshing, corotating, or counterrotating screws. Screws may also be nonintermeshing, of constant depth, or conical in shape.
The barrel of the extruder is normally a long tube. It is made of a special hard steel alloy to provide
resistance against wear and corrosion and has thickness sufficient to withstand high internal pressure
(about 10,000 to 20,000 psi) without failure. The barrel is equipped with systems for both heat input
and extraction. The heaters are arranged in zones and are usually made of cast aluminum. Both halves
of the heater are clamped to the barrel to ensure intimate thermal contact. Heat is extracted either by
air or a liquid coolant. The simplest design is to mount the coolant or blower under each heater. The
heaters, operated manually or by automatic controllers, keep each zone of the extruders at a defined
temperature. The use of microprocessors for temperature control has proved increasingly successful.
The feed throat at the end of the barrel is generally jacketed for cold water circulation. This keeps the
surfaces of the barrel section below the temperature that could cause premature softening of the polymer,
which then sticks to and rotates with the screw. The resulting material buildup could seal off the screw
channel and prevent the forward movement of the polymer — a phenomenon known as bridging.
As can be seen in Figure 11.2, the extruder is also equipped with a gearbox, thrust-bearing mechanism,
and motor. The screw derives its power for rotation from an electric motor. Power requirements are in
the range of 1 hp per 5 to 10 lb per hour of material passing through the extruder. The electric motor
has a speed of about 1700 rpm, which is too fast for a direct connection to the screw, which has speeds
in the range 20 to 200 rpm. It is therefore necessary to employ a gear reducer, which provides varying
screw speeds by matching the speed of the drive motor and the rotating screw. Variable speed units can
usually provide a change of screw speed from 8 to 1. This enables control of extruder output since there
is a direct relation between the extruder pumping rate and the screw speed.
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285
UNIT OPERATIONS IN POLYMER PROCESSING
The extruder screw is designed to develop the pressure required to pump the molten polymer through
the die. This pressure also acts on the screw. Since the thrust bearing mechanism supports the drive
mechanism into which fits the shank of the screw, the thrust-bearing mechanism resists the axial thrust
exerted by the molten polymer on the screw. Pressures of up to 2000 psi can be developed in many
extruder operations.
Some polymers contain unreacted volatile monomer, moisture, and entrapped gases, which when
emitted during the extrusion process could potentially contribute to poor product quality. Some extruders
are provided with vents to remove these volatiles from the melt before it reaches the die. The vent is a
hole in the barrel about screw diameter size, and located about three-fourths of the distance from the
feed throat to the front of the barrel. In some cases, a vacuum is attached to the vent to facilitate gas
removal.
The screw for a vented extruder, referred to as a two-stage screw, has a special design. The screw
section under the vent has a deep channel. The section of screw before the vent can be considered a
normal screw with feed, transition, and metering sections. The section following the vent consists of a
short transition zone and a metering section. The channels of the metering section after the vent are
deeper than those of the metering section before the vent. This, coupled with the relatively higher channel
depth under the vent, means that the channel is partially filled at and beyond the vent section and aids
gas removal.
Example 11.1: The barrel of a 6-in. extruder must be able to withstand an internal pressure of 10,000 psi
without elastic deformation of 0.15%, at which the alloy will crack in tension. If the barrel material is
made of hard steel, estimate the thickness of the tube.
Solution: The stresses existing in the walls of a cylindrical vessel under pressure can be reduced to
(1) the tangential or hoop stress σh, (2) the radial stress σr, and (3) the longitudinal stress σl. For a
cylinder under internal pressure, σh and σ l are tensile stresses while σr is a compressive stress. σl is
generally smaller (about half the magnitude) than σh and is not considered in connection vessel failure.
For a thick-walled vessel (i.e., where ro/ri = R > 1.2):
σn =
(
)
pi R 2 + 1
R2 − 1
where Pi = internal pressure. The maximum allowable hoop stress σh = Eε.
E for hard alloy = 30 × 10 6 psi
ε = strain
σ n = 0.0015 × 30 × 10 6
= 4.5 × 10 4 psi
4.5 × 10 4 =
(
(
)
10 4 R 2 + 1
R2 − 1
)
4.5 R 2 − 1 = R 2 + 1
R 2 (4.5 − 1) = 5.5
R = 1.25
ro = 1.25ri = 7.5 in.
ro − ri = 1.5 in.
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POLYMER SCIENCE AND TECHNOLOGY
B. EXTRUSION PROCESSES
The die shapes the polymer extrudate into the desired article. There are a large number of extrusion
processes, the simplest of which is compounding. In one variation of extrusion, the die has a series of
holes and as the polymer exudes from these holes it is cooled and cut into pellets. In pipe extrusion, the
extrudate exiting the die is vacuum-sized and quenched in a water trough. Polymer melts, being viscoelastic, recover the stored elastic energy as they emerge from the die — a process known as die swell.
In pipe extrusion, die swell must be controlled to ensure that the pipe dimensions meet standard codes.
Profile extrusion is similar to pipe extrusion. However, unlike pipes whose shape is round, hollow, and
uniform, the shapes of profiles depend on the end product, which usually is the raw material for
downstream processing.
Fibers of various gauges and lengths are obtained in fiber extrusion. These can range from monofilament such as fishing lines to hundreds of continuous filaments extruded from the same die and drawn
to hair-sized thickness. The continuous filaments may be crimped for added bulk or made into staples.
Almost all electrical wire and cable insulation is currently done by covering the wires or cables with
one or more layers of thermoplastic insulation. These take different forms: wire strands may be covered
with several layers by successive insulation; multiple preinsulated wires may be covered to form a single
cable; or several bore wires may be drawn through the die simultaneously and covered with insulation
forming a ribbon cable. In coextrusion, two or more different materials or the same material with two
different colors are extruded through the same die so that one material flows over and coats the other.
Coextrusion is used to achieve different objectives: a solid cap may be extruded over a foamed core for
overall weight reduction or to obtain insulation in addition to a serviceable outer surface; for materials
with surfaces sensitive to color, a virgin cap may be extruded over reground, or several different materials
may be coextruded so that each material contributes a derived property to the end product.
III.
INJECTION MOLDING2-4
Injection molding is one of the processing techniques for converting thermoplastics, and recently,
thermosetting materials, from the pellet or powder form into a variety of useful products. Forks, spoons,
computer, television, and radio cabinets, to mention just a few, are some of these products. Simply,
injection molding consists of heating the pellet or powder until it melts. The melt is then injected into
and held in a cooled mold under pressure until the material solidifies. The mold opens and the product
is ejected. The injection molding machine must, therefore, perform essentially three functions:
1. Melt the plastic so that it can flow under pressure.
2. Inject the molten material into the mold.
3. Hold the melt in the cold mold while it solidifies and then eject the solid plastic.
These functions must be performed automatically under conditions that ideally should result in a
high quality and cost-effective part. Injection molding machines have two principal components to
perform the cyclical steps in the injection molding process. These are the injection unit and the clamp
unit (Figure 11.4). We now describe the operation of the various units of the injection molding machine
that perform these functions.
A. THE INJECTION UNIT
The injection unit essentially has two functions: melt the pellet or powder and then inject the melt into
the mold. It consists of the hopper, a device for feeding process material; a heated cylinder or chamber
where the material is melted; and a device for injecting the molten material into the mold. In the early
days when the amount of processed material was relatively small, the two functions of melting and
injecting the polymer were accomplished by using a simple plunger machine (Figure 11.5). In this
system, a measured volume of the plastic material is delivered into the heated cylinder from the hopper
while the ram is retracted. At the beginning of the injection cycle, the plunger pushes forward and forces
the material through the heated cylinder compacting it tightly behind and over the centrally located
spreader or torpedo. The material is melted by heat convection and conduction. The sustained forward
motion of the plunger forces the melt through the nozzle of the cylinder into the mold.
In the plunger-type machine, material flow in the cylinder is essentially laminar. Consequently there
is hardly any mixing in this system and, as such, large temperature gradients exist in the melt, and color
Copyright 2000 by CRC Press LLC
UNIT OPERATIONS IN POLYMER PROCESSING
287
Figure 11.4 Major parts of a typical injection-molding machine. (From Weir, C.L., Introduction to Molding,
Society of Plastics Engineers, CT, 1975. With permission.)
Figure 11.5 Schematic diagram of a plunger-type injection molding machine. (From Weir, C.L., Introduction to
Molding, Society of Plastics Engineers, CT, 1975. With permission.)
blending is thus problematic. Also, as a result of the friction between the cold resin pellets in the
neighborhood of the hopper and the barrel walls, a considerable loss of pressure, up to 80% of the total
ram pressure, occurs. This necessitates long injection time. As indicated above, the resin is melted by
heat conduction from the walls of the cylinder and the resin itself. Since plastics are poor heat conductors,
high cylinder temperatures are required to achieve fast resin plasticization. This can result in the
degradation of the material. To avoid such possible material deterioration, the heating of the cylinder is
limited, and this also limits the plasticizing capacity of plunger-type injection machines.
Today, the plunger has been replaced almost totally by the plasticating screw. As described in Section
II, the screw consists basically of three sections: the feed, the transition zone, and the metering section.
Melting normally starts halfway down the length of the screw at which point the depth of the screw
flights decreases initiating the compression of the melt. This marks the beginning of the transition zone,
which terminates at that metering section — the point where the depth of the flights is minimum. A
Copyright 2000 by CRC Press LLC
288
POLYMER SCIENCE AND TECHNOLOGY
Figure 11.6 Schematic drawing of injection end of two-stage screw-plunger machine. (From Kaufman, H.S.
and Falcetta, J.J., Eds., Introduction to Polymer Science and Technology, John Wiley & Sons, New York, 1977.
With permission.)
number of screw configurations exist, including the mixing screw, the marblizing screw, and that with
a decompression zone geometry to permit venting and ridding the melted material of volatiles.
There are essentially two methods of using the plasticating screw. The first is the screw-plunger
system also called the two-stage or screw-pot system. The screw rotates in the heated barrel and
consequently plasticizes the polymer material. The plasticized material is then transferred into a second
heated cylinder from which it is injected into the mold by a plunger. Figure 11.6 shows the basic features
of a screw-pot injection molding machine.
In the second approach developed in the mid-1950s, a reciprocating screw is employed. As the screw
rotates, it picks up the material from the hopper. The material is melted primarily by the shearing action
of the screw on the resin. The electrical heating bands attached to the barrel essentially provide start-up
and compensate for heat losses. The rotation of the screw moves the melted material forward ahead of the
screw. The pumping action of the screw generates back-pressure, which forces back the screw, the screw
drive system, and the hydraulic motor when enough melted material required for a single shot has accumulated in front of the screw. As the screw moves backward, it continues to turn until it hits a limit switch,
which stops the rotation and backward movement. The location of the limit switch is adjustable and
determines the shot size. Meanwhile the clamp ram has moved forward with the movable platen and closed
the mold. When the backward movement of the screw is stopped, the hydraulic cylinders bring the screw
forward rapidly and inject the melted material through the nozzle into the mold cavity. A ball check or
check ring on the end of the screw (valve) prevents the melt from leaking into the flights of the screw
during injection. Consequently, most of the melt is forced into the mold (Figure 11.7). The mold is cooled
and the part solidifies. At the end of the predetermined period, the mold opens as the movable platen returns
to its initial position and the part is ejected. The screw starts rotating, beginning the next cycle.
The plasticating screw-type machines obviate most of the problems associated with the plunger
system. The resin is heated mainly by viscous heat generation as opposed to thermal conduction from
cylinder walls. In addition, in contrast to the block of material in the barrel in plunger-type machines,
only a thin layer of material exists between the screw and the barrel walls and thus the resin is heated
faster. Also in the screw-type machines, the melt is thoroughly mixed. Consequently, these machines
produce a melt with a more uniform temperature and homogeneous color, high injection pressures, faster
injection speeds and shorter cycles, and therefore higher production rates.
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UNIT OPERATIONS IN POLYMER PROCESSING
289
Figure 11.7 Schematic diagrams of a screw-type injection-molding machine: (top) screw retracted, before
injection; (bottom) screw forward, at end of shot. (From Weir, C.L., Introduction to Molding, Society of Plastics
Engineers, CT, 1975. With permission.)
Example 11.2: Explain the following observations:
a. Screw-machine-made end products generally have better physical properties, e.g., lower shrinkage, than the same products made from plunger-type machines.
b. Polymers like PVC and HDPE are easier to handle in screw machines than plunger machines.
Solution:
a. To a very large extent, high injection pressures create internal stresses in the molded part. The
resulting part, consequently, has inferior physical properties and is vulnerable to rapid quality
deterioration in use. Low injection pressures are therefore desirable. In plunger-type machines,
there is usually excessive pressure loss. Therefore, injection pressures are necessarily high in
these machines. In contrast, the same parts can be molded in reciprocating screw machines with
much lower pressures. Indeed, pressures can be reduced to as much as one-half to two-thirds
those of the plunger-type machines, particularly with high viscosity materials. Thus, parts
produced in reciprocating screw machines have better physical properties than the same part
made from a plunger-type machine.
b. Although screw machines are more complicated than plunger type machines, they are also more
flexible and their operation and adjustment to varying molding conditions are simpler. Resin
hang-up and the attendant decomposition and the loss of physical properties due to overheating
occur very rarely in screw machines. Also, it is much easier to keep the melt temperature within
limits by the regulation of screw speed and barrel heating. Thus heat-sensitive polymers like
PVC are easier to handle in screw-type machines. In addition, some polymers require a greater
heat input to effect plasticization. For example, the heat requirement for high-density polyethylene (HDPE) is about 310 Btu/lb. As a result of the greater shearing action and the attendant
viscous heat generated in screw-type machines, polymers like HDPE are easier to handle in
these machines.
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POLYMER SCIENCE AND TECHNOLOGY
B. THE PLASTICIZING SCREW
Given the relative advantages of screw type machines over plunger-type machines, it is instructive to
examine the plasticizing screw more closely. The rotation of the screw generates a torque related to the
power requirements according to Equation 11.1.
Horsepower ( hp) =
[Torque (ft-lb)] [Rpm]
5252
(11.1)
where rpm is revolution per minute of the screw. Recall that torque is the product of the tangential force
and the distance from the center of the rotating member. It is clear from Equation 11.1 that there is an
inverse relation between the speed and the torque generated by the screw. Generally, a motor of a given
horsepower has a fixed speed. Consequently, a unit with a higher torque (lower speed) has a larger frame
than that with a lower torque. The screw can be attached to the motor directly or through a gear train.
The latter provides an alternative mechanism of changing the speed and therefore the torque generated
by the screw. A range of torque values is desirable for handling various materials due to the different
processing characteristics of polymers. For example, much higher torque is required to plasticize polycarbonate than polystyrene.
For a given horsepower, the slower the screw speed, the higher the torque developed. However, the
yield strength, and therefore the torque a screw can sustain, varies as the cube of the root diameter of
the screw. It follows that if the torque generated is too high, the yield strength of the screw material
may be exceeded, causing a shearing of the screw. On the other hand, if the screw speed (shear rate) is
too high, the material may be degraded. As indicated earlier, plasticization of the polymer is a result of
the viscous energy developed from the work done by the rotating screw. In other words, the screw output
is related to the energy developed from the shearing forces. Therefore, if different screw designs have
similar efficiency, the maximum screw output is determined largely by the power rating of the screw.
Example 11.3: A plasticizing screw driven by a 50-hp motor is used to raise the temperature of the
polymer feed from 25°C to the processing temperature. Assuming a mechanical efficiency of 60% and
neglecting the energy required to pump the polymer and that from the heating bands, estimate the screw
output if:
a. The processing temperature is 440°F and the specific heat of the polymer material is 0.8 Btu/lb °F.
b. The processing temperature is 500°F and the specific heat of the polymer material is 0.8 Btu/lb °F.
c. The processing temperature is 500°F and the specific heat of the polymer material is 0.4 Btu/lb °F.
Solution: a. Since the energy from the heating bands and the energy required to pump the polymer are
to be neglected, the power supplied by the motor is converted into work by the screw. The viscous energy
dissipated goes into melting the polymer.
Power = Cp Q (Tm – Tf)
Tm = temp of polymer melt
Tf = temp of polymer feed
Cp = specific heat of polymer material (assumed constant)
Q = Screw output (in mass/time)
If Power = hp, Q = lb/h, then since 1 Btu/h = 0.0004 hp, hp = 0.0004 Cp Q [Tm – Tf].
Q=
=
hp
(lb h)
0.0004 C p (Tm − Tf )
50 × 0.6
0.0004 × 0.8 (440 − 72)
= 255 lb h
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UNIT OPERATIONS IN POLYMER PROCESSING
Q=
291
50 × 0.6
0.0004 × 0.8 (500 − 72)
= 219 lb h
Q=
50 × 0.6
0.0004 × 0.4 (500 − 72)
= 438 lb h
Example 11.4: Explain the following observations:
a. It is generally necessary to mold plastics at the lowest possible melt temperature.
b. If two screws with different diameters but with the same L/D ratio are operated under identical
conditions, the polymer material has a longer residence time in the larger diameter screw machine
even though the machine output is the same. However, it is preferable sometimes to operate
machines with the larger diameter screws.
Solution:
a. From Problem 11.3 it is obvious that raising the material processing temperature lowers output.
Also, it increases the cycle time because the mold cooling time is longer. Therefore, to maximize
output and increase overall productivity, it is best to mold at the lowest possible melt temperature.
b. Productivity or machine output is the primary concern of injection molders. However, high screw
speeds and hence high productivity can result in material degradation and poor product quality.
Consequently, it is generally prudent to operate machines at reasonably slower speeds. But
because of the inverse relation between torque and screw speed (Equation 11.1) for the same
power input drive, the magnitude of the torque developed by the smaller diameter screw will be
higher than that of the larger diameter screw and may be high enough to shear the screw.
Therefore, even when machine output is the same, larger diameter screws may be preferable to
smaller ones to prevent possible screw damage.
C. THE HEATING CYLINDER
The heating or plasticizing cylinder of the injection molding machine is the primary element of the
machine. It is here that the polymer material is softened or conditioned for injection into the mold cavity
where it is shaped. The temperature and pressure of the melt as it leaves the cylinder nozzle into the
mold cavity are two important variables that determine product quality. Cylinders are generally rated
on the basis of their plasticizing capacity which is the rate at which the given cylinder can condition a
given polymer material into a state suitable for injection. As may be expected, for a given cylinder, this
varies with the particular plastic material being processed because the molding (softening or melting)
temperature, specific heat, thermal conductivity, and specific gravity — all of which contribute to the
complex heat generation and transfer processes which occur during processing — differ for various
materials. Consequently, the plasticizing capacity of a machine is rated conventionally on the basis of
one material — general purpose polystyrene, which is taken as the standard. The machine capacity with
respect to other materials is then related to this standard material using the relative specific gravities of
the two materials.
Table 11.2 gives the specific gravities of some plastic materials, while Table 11.3 shows the appropriate cylinder heater inputs and power requirements of typical machine sizes.
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POLYMER SCIENCE AND TECHNOLOGY
Table 11.2 Specific Gravities of Some Polymers
Resin
Specific Gravity
ABS
Acetal
Cellulose acetate
Cellulose acetate butyrate
Cellulose propionate
Ethyl cellose
Nylon
Polycarbonate
Polyethylene
Low density
High density
Poly(methyl methacrylate)
Polypropylene
Polystyrene
General purpose
High impact
Polyvinyl chloride
Plasticized
Unplasticized
1.04
1.42
1.27
1.19
1.21
1.10
1.14
1.20
0.92
0.95
1.19
0.90
1.05
1.04
1.20
From Weir, C.L., Introduction to Molding, Society of Plastics
Engineers, CT, 1975. With permission.
Table 11.3 Cylinder-Heater Inputs and Power
Requirements Typical Machine Sizes
Shot
Size (oz)
Plasticizing
Capacity (lb/h)
Approximate
Heat Input (kW)
Screw Power
Input (hp)
1
3
6
9
12
24
40
60
80
100
150
200
225
350
17.5
45
80
125
220
250
400
440
700
750
850
1000
1500
1800
2.0–2.5
2.8–4.1
4.4–5.2
8.5–9.0
9.0–10.5
10.6–12.0
15–20
20–25
25–30
30–40
40–50
60–70
68–75
75–100
3.5
5
10
15
25
40
50
60
75
90
100
125
150
200
From Weir, C.L., Introduction to Molding, Society of Plastics Engineers,
CT, 1975. With permission.
Example 11.5: An injection molding machine is rated 100 oz per shot. What is the plasticizing capacity
(lb/h) of this machine if the total cycle time is 30 s? What is the average residence time of the material
in the heating cylinder for an inventory weight of 50 oz? Estimate the plasticizing capacity of the heating
cylinder for low density polyethylene.
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UNIT OPERATIONS IN POLYMER PROCESSING
Solution: The machine output or plasticizing capacity, Q, is the ratio of the shot weight W to the cycle
time tc.
Q=
W (oz)
t c (s)
W (oz) 1 lb 3600 s
t c (s) 16 oz 1 h
= 225
Q=
W lb
tc h
225 × 100 oz
= 750 lb h
30
Residence or contact time in the heating cylinder, tc , is given by the inventory weight, Iw , divided by
the machine output Q.
t=
Iw
Q
I oz 225 × 50
t (s) = 225 w
= 15 s
=
750
Q lb h
Plasticizing capacity for LDPE =
0.93
× 750
1.05
= 657 lb h
D. THE CLAMP UNIT
The clamp unit or press end of the injection molding machine performs three functions: opens and closes
the mold at appropriate times during the molding cycle; ejects the molded part; and provides enough
pressure to prevent the mold from opening due to the pressure developed in the mold cavity as it is filled
with the melt by the injection unit. Injection pressures in the plasticating cylinder can range from
15,000 to 20,000 psi in a given system. As a result of possible pressure drops in the cylinder and the
nozzle, the effective pressure within the mold cavity may be reduced to 25 to 50% of this value.
Consequently, the force needed to resist the premature opening of the mold and obtain an acceptable
part can be quite large. For example, assuming a 50% pressure drop and using the upper possible pressure
in the cylinder stated above, the melt pressure in the mold cavity becomes 10,000 psi. This translates
into a clamp force of 5 tons/in2 of the projected area of the part. However, as a result of greater degree
of homogenization achievable in screw-type machines, the clamp tonnage required in these machines is
generally less than in plunger-type machines.
The halves of the mold are attached to the platens, one of which is stationary and one of which moves
as the clamp mechanism is opened or closed. Molds are generally designed so that the ejection side of
the mold (mold core) is on the movable platen and the injection side of the mold (mold cavity) is on
the stationary platen, which must provide an entry for the nozzle of the plasticizing chambers. When
the mold opens, the movable platen must be moved sufficiently for the part to be ejected. Shrinkage
usually accompanies part solidification and this results in the part sticking to the core as the mold opens.
Consequently, the movable platen is provided with an ejector or knockout system to eject the part. The
ejector usually consists of a hydraulically actuated ejector plate mounted off the back face of the movable
platen. The ejector system causes the knockout plate to change its location relative to the rest of the
mold. The ejector pins attached to this plate push against the molded part and eject it from the mold.
The force required to eject the part is usually less than 1% of the nominal clamp force; its magnitude
depends on the part geometry, material, and packing pressure.
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Figure 11.8 Elements of hydraulic system.
There are essentially two categories of clamp systems for moving and locking the movable platen.
These are the mechanical (toggle) and hydraulic systems. The toggle system, which is used mainly in
small tonnage (about 500 tons) equipment, is relatively inexpensive but requires good maintenance
because wear reduces the clamping force. Hydraulic systems, used primarily in medium and large
equipment (1500 to 2000 tons), are relatively slow acting but have the advantage of providing virtually
limitless pressure selection, which can be followed continuously with a pressure gauge. Machine flexibility permits the necessary adjustment to match mold thickness and settings.
E. AUXILIARY SYSTEMS
The operation of the injection and clamp units and other components of the injection molding machine
(opening and closing of the mold and melting and injection of the polymer material) requires power,
which is supplied by an electric motor. The orderly delivery of this power depends on auxiliary systems:
the hydraulic and control systems. The hydraulic system, the muscle for most machines, transmits and
controls the power from the electric motor to the various parts of the machine. Machine functions are
regulated by a careful control of the flow, direction, and pressure of the hydraulic fluid. The elements
of the hydraulic system for most injection molding machines are essentially the same: fluid reservoir,
pumps, valves, cylinders, hydraulic motors, and lines (Figure 11.8).
The pump driven by the electric motor draws oil from the reservoir and delivers it to the system
through the suction filter. The restriction of oil flow by the control valve and/or the resistance to movement
of the cylinder compress the oil and lead to pressure buildup. A pressure buildup of appropriate magnitude
drives the hydraulic cylinder, and if this pressure reaches that set for the relief valve, the valve opens
and excess fluid is bypassed to the reservoir. The opening of the relief valve decompresses the oil,
converting the excess pressure energy into heat, which raises the oil temperature. The oil is cooled by
passing it through a heat exchanger.
The injection molding operation involves a sequence of carefully ordered events, e.g., precise heat
control and appropriate timing of injection pressures and cycles. The control system is the nerve center
responsible for the orderly execution of the various machine functions. Over the years, the control system
has developed from a collection of relays that perform logic, plug-in timers for timing functions, and
plug-in temperature controllers for cylinder temperature regulation to solid-state circuitry for the control
of these functions to the current microcomputer control, which has greatly enhanced process control.
F. THE INJECTION MOLD
The injection mold is a series of steel plates, which when assembled produces the cavity that defines
the shape of the molded part. Conventional molds consist of the mold frame, components, runners,
cooling channels, and ejector system. The mold frame is a collection of steel plates that contain mold
components and runners, cooling and ejection systems. Components are parts inserted into either bored
holes or cutout pockets in the mold frame. The polymer melt enters into the mold cavity or cavities
through the runners, which are passages cut into the mold frame. The hot polymer material in the mold
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Figure 11.9 Exploded view of a six-plate mold base. (From Weir, C.L., Introduction to Molding, Society of
Plastics Engineers, CT, 1975. With permission.)
is cooled by a coolant, usually water, which circulates through the cooling channels drilled at strategic
locations into the mold frame and components for proper mold temperature control. When the material
has cooled (hardened) sufficiently, the mold opens and the hardened part is removed from the mold by
the ejector system.
Since the quality of the molded part depends to a large extent on the mold, it is essential to expand
on the functions of the various parts of the mold. Details of mold components are shown in Figure 11.9.
The register or retainer ring, which is fitted into the stationary platen, aligns the mold with the cylinder
nozzle. Sometimes, the retainer ring, as the name suggests, also acts as the retainer of the sprue bushing
within the mold. The sprue is the pathway through which the molten polymer material is introduced
into the mold cavity area or the runner system from the cylinder nozzle. The sprue is generally made
as small as practical since large diameter sprues have been known to require longer cooling time and
consequently prolong cycle time beyond what is usually necessary for part thickness. In some newer
mold designs, the sprue is completely eliminated.
The front clamp plate, which houses the retaining ring and sprue bushings, is used to support the
stationary half of the mold, including the front cavity plate. The cavity (cavities) of the part to be molded
is contained in the cavity plate in either of two ways. In one case the cavity is drilled directly into the
steel plate, and in the other the cavity plate provides sockets for the insertion of cavities that have been
constructed separately. The rear cavity plate, mounted on the rear support plate, contains the core section
of the molded part or second half of the mold cavity. The leader (guide) pins and guide pin bushings
are housed in the front cavity and rear cavity plates, respectively. The passage of the guide pins through
the bushings ensures that the plates are properly aligned during closing of the mold. The knockout and
reset pins are mounted on a series of plates that compose the ejector plate. At appropriate times during
the molding cycle, these pins pass through holes in the cavity and cavity retainer plates and make contact
with either the molded part to effect its ejection or the stationary cavity plate to initiate the movement
of the pin plate back in readiness for the next injection shot. The movable half of the mold is anchored
to the movable platen by the rear clamp plate.
As the mold cavity is filled with polymer melt, the pressure increase within the cavity can produce
stresses of up to 10,000 to 180,000 psi in the mold cavity material. The resulting deformation is substantial
but can be accommodated provided the elastic limit of the material is not exceeded. However, where
the dimensional tolerance of the molded part is critical, it is imperative that the mold material modulus
is sufficiently stiff to ensure part dimensional accuracy. Maintenance of proper temperature of the mold
cavity and core is also necessary in this respect as well as for the production of a molded part with good
physical and mechanical properties. Dimensional accuracy of the part also demands that the dimensions
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of the mold be slightly larger than those of the molded part so as to compensate for the shrinkage that
normally accompanies the solidification of a polymer melt. Finally, a good surface appearance of the
molded part such as luster and smoothness requires a polished mold cavity.
Runners are the channels through which the polymer melt is fed into the mold cavities from the
cylinder nozzle. In a multicavity mold, it is necessary to fill all the mold cavities simultaneously and
uniformly. Control of the size of the runners provides a means of controlling the flow resistance and
balancing the flow into the mold cavities. In most multicavity molds, the runners form part of the mold
frame. Consequently, the ejected part is accompanied by the runner system, which must be removed
and, in the case of thermoplastics, reground for reuse. The use of the hot runner mold whereby the
runner channels are heated to keep the polymer in the molten state, eliminates this need for plastic runner
separation and avoids possible generation of scrap material. With proper machine operation, a hot runner
mold requires a smaller amount of melt per shot than an equivalent cold runner mold, leading to reduced
injection time and faster cycles.
Runners feed the mold cavities through the gates. The gate of an injection mold is one of its most
vital components. The size, type, and location of the gate affect production rate and the quality of the
molded part. As the polymer melt flows into the mold cavity, pressure in the cavity builds up and, if not
controlled, can develop an undesirable magnitude. Control of pressure in the cavity requires that the
gate freeze as soon as sufficient material has been fed into the mold cavity and only to the point where
available pressure is sufficient to cause resumption and maintenance of adequate melt flow. If the gate
freezes prematurely, the mold will not be completely filled and/or adequately packed to compensate for
shrinkage. This produces a warped and unacceptable part. If, on the other hand, the mold cavity is filled
and the gate takes too long to freeze, the resultant back pressure causes the still-hot polymer melt to
flow back out of the mold into the nozzle and cylinder when the ram or screw is withdrawn.
Finally, to conclude this section, we briefly address mold cooling and the associated shrinkage and
possible warpage of the molded part. As we discussed earlier, to complete the injection cycle the polymer
melt injected into the mold cavity must be cooled to such a stage that the solidified part can be ejected.
Ideally the temperature distribution of the mold surface should be uniform so as to ensure uniform
temperature reduction, even shrinkage, and reduced warpage tendency of the molded part. This demands
that the size and distribution of the cooling lines be such that cooling rate is highest near the gating and
sprue where the melt temperature is highest and lowest in areas farthest from these points.
You will recall from our discussion in Chapter 5 that polymers undergo a reduction in specific volume
(shrinkage) as they undergo a phase transformation from the molten to the solid state. The severity of this
dimensional change varies with the nature of the polymer: crystalline polymers are more seriously affected
than amorphous polymers. Therefore, a certain amount of mold shrinkage is inevitable in polymer molding
operations. For most applications, this can be accommodated by proper mold design. However, for sophisticated parts and those parts that require close tolerance, mold shrinkage assumes an added importance.
This must be addressed not only through mold design but by a more careful control of operating conditions.
Polymer molecules generally assume a random orientation in the melt. As the melt is forced through the
gate during mold filling, polymer molecules tend to lose this random orientation and align themselves along
the direction of flow. The polymer material that comes into contact with the much colder mold surface chills
rapidly and becomes frozen in place. Meanwhile, the melt not yet in contact with the mold surface continues
to move. The frictional forces (shearing stresses) between the moving and stationary polymer material, which
are generally high, stretch the polymer molecules in the direction of flow. This orientation of the polymer
molecules results in orientation strains or built-in stresses in the molded part. The relaxation of these stresses
in the ejected part leads to a further or postmold shrinkage of the molded part. Uneven shrinkage causes the
part to warp, particularly in large, thick, and flat molded articles and may lead to part rejection.
Example 11.6: Explain the following observations:
a. Nonferrous metals are used in injection molds for cavity and core components but must be
properly supported on steel forms.
b. The dimensions of the mold cavity are generally made larger than those of the molded part to
allow for the shrinkage that occurs when the molten polymer cools. Typical shrinkage allowances
for some plastics are shown in Table E11.6.
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Table E11.6 Shrinkage Allowances for Some Polymers
Polymer
Poly(methyl methacrylate)
Nylon
Polyethylene
Polystyrene
Shrinkage Allowance (in./in.)
0.006
0.023
0.023
0.006
Solution:
a. The surface finish of the mold cavity contributes to the surface appearance (luster and smoothness)
of an injection-molded part. Consequently, mold cavities must be properly polished. Nonferrous
metals are generally softer and easier to polish than hardened steel. To ensure the dimensional
accuracy of the finished part, the surface temperature of injection mold cavities and cores must
be maintained at appropriate temperatures during operation. Nonferrous metals with their high
thermal conductivities ensure uniform temperature distribution. However, nonferrous metals are
not sufficiently stiff and therefore are susceptible to permanent deformation that may affect not
only the surface appearance but also the dimensional accuracy of the molded part. Consequently,
it is often necessary to support nonferrous metals on the much stiffer hardened steel.
b. On cooling from the viscous state (melt), crystalline polymers shrink more than amorphous polymers
because the process of crystallization involves a considerable contraction of volume. Poly(methyl
methacrylate) and polystyrene are amorphous, while nylon and polyethylene are crystalline.
Example 11.7: Explain and comment on the following observation: prevention of excessive shrinkage
of a molded part requires molding at low injection temperatures, running a cold mold, or operating at
high melt pressures, whereas reduction of part warpage calls for maximum injection pressure.
Solution: Polymeric materials exhibit a marked decrease in specific volume (shrink) as they are cooled
from the melt. Crystalline materials shrink more than amorphous materials as a result of the more ordered
molecular arrangement. The amount of shrinkage is determined by the rate of quenching or temperature
drop from the injection temperature to the mold temperature. Lowering the injection temperature reduces
this temperature drop and consequently reduces shrinkage. High mold temperature allows the polymer
melt to cool more slowly, promotes Brownian movement, and thus facilitates a more ordered molecular
arrangement. On the other hand, running a cold mold essentially freezes in the amorphous arrangement
of the melt. This in effect translates into a higher specific volume or a reduction in part shrinkage.
As the hot material comes into contact with the cold mold surface, it solidifies and shrinks. Operating
at high injection pressures results in a compression of the material and therefore permits more material
to flow into the mold cavity and compensate for the shrinkage. The increased mold packing that is
associated with high injection pressures means that more material can be packed into a given volume,
therefore reducing shrinkage.
Warpage is due largely to the internal stress developed in the molded part during the molding operation.
Molding at high injection temperatures tends to diminish the elastic memory of the polymer melt and
thus reduce the tendency to create stresses that might cause warpage. As indicated above, the faster the
cooling rate of the polymer melt, the more disorder the molecular arrangement of the resulting solidified
material and consequently the greater the built-in stresses in the molded part. Operating high mold
temperatures allows the molecular rearrangement that is required for stress relief and thus a reduction
of the internal stresses that cause part warpage. Low injection pressures lead to the generation of much
lower orientation strain.
It is obvious from this discussion that procedures for reducing part shrinkage work in direct opposition
to those required for preventing its warpage. Therefore, the injection molding of a trouble-free part
requires a careful balancing of these operating conditions.
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IV.
BLOW MOLDING4,5,8
Blow molding is a process used extensively for the production of bottles and other hollow plastic items
with thin walls. Blow-molded objects may range in size from less than 1 oz to a few hundred gallons.
A. PROCESS DESCRIPTION
The blow molding process consists of a sequence of steps leading to the production of a hollow tube or
parison from a molten thermoplastic resin. This is then entrapped between the two halves of a mold of
the desired shape. Air, usually at about 100 psi, is blown into the soft parison, expanding it against the
contours of the cold mold cavity. The part is cooled and removed from the mold, and where necessary
the excess plastic material or flash accompanying the molded part is trimmed and reclaimed for reuse.
The blow molding process therefore involves essentially two properly synchronized operations:
parison formation from the plastic material and blowing the parison into the shape of the desired part.
There are two techniques for plasticizing the resin for parison formation. These are extrusion blow
molding (which is the most common method and which is characterized by scrap production) and
injection blow molding. The latter process is versatile and scrap free and is beginning to be more
understood and accepted by processors.
B. EXTRUSION BLOW MOLDING
In extrusion blow molding, an extruder, as described in Section II, is used to plasticize the resin and
form the parison. The process may be continuous or intermittent. In the continuous process, a continuous
parison is formed at a rate synchronized with the rates of part blowing, cooling, and removal. Two
general mold clamp mechanisms are used for part formation from the extruded parison. In the first
arrangement or shuttle system, the blowing station is situated on one or both sides of the extruder. As
soon as an appropriate length of parison is extruded, the clamp mechanism moves from the blowing
station to a position under the die head, captures and cuts the parison, and then returns to the blowing
station for part blowing, cooling, and removal. This ensures that there is no interference with parison
formation. In the second or rotary system, a number of clamping stations are mounted on a vertical or
horizontal wheel. As the wheel rotates at a predetermined rate, blowing stations successively pass the
parison head(s) where it is entrapped for subsequent part formation. In this case, parison entrapment
and blowing, part cooling, and removal occur simultaneously in a number of adjacent blowing stations.
In the intermittent extrusion process, molding, cooling, and part removal take place under the extrusion
head. An extruder system, which may be of the reciprocating screw, ram accumulator, or accumulator
head type, extrudes the parison in a downward direction where it is captured at the proper time between
the two halves of the mold. The part is then formed and ejected and a new cycle begins (Figure 11.10).
As the name suggests, in the intermittent extrusion blow molding process, parison formation is not
continuous. For example, with the reciprocating screw machine, after the parison is extruded, melt is
accumulated in front of the screw causing a retraction of the screw. After the molded part has cooled
and the mold opens and ejects the part, the screw is immediately pushed forward by hydraulic pressure,
forcing the melt into the die to initiate the formation of the next parison.
Figure 11.10 Schematic of the blowing stage. (a) The molten, hollow tube — the parison or preform — is
placed between the halves of the mold; (b) the mold closes around the parison; (c) the parison, still molten, is
pinched off and inflated by an air blast that forces its wall against the inside contours of the cooled mold; (d) when
the piece has cooled enough to have become solid, the mold is opened and the finished piece is ejected. (From
Kaufman, H.S. and Falcetta, J.J., Eds., Introduction to Polymer Science and Technologies, John Wiley & Sons,
New York, 1977. With permission.)
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C. INJECTION BLOW MOLDING
The injection blow molding process is a noncontinuous cyclic process consisting essentially of two
phases. In the first phase, a preform is molded by injecting melted plastic into a steel mold cavity where
it is kept hot and conditioned. In the second or subsequent phase, the preform is metered into the blow
mold where the blowing operation takes place to form the final part. The major advantages of injection
blow molding are the quality of the molded part and productivity. There is no flash production. Therefore,
the molded part neither has a pinch-off scar from flash nor requires additional trimming or other finishing
steps for waste retrieval. Also, the molded parts show hardly any variation in weight, wall thickness,
and volume from the accurately molded preform. However, only blow-molded parts with limited size
and shape and without handles are feasible with the injection blow molding process.
V. ROTATIONAL MOLDING4,5
A. PROCESS DESCRIPTION
Rotational molding is a process used for producing hollow, seamless products having heavy and/or
complex shapes. In rotational molding a premeasured amount of powder or liquid polymer is placed in
the bottom half of the mold, and the two halves of the mold are locked together mechanically. The mold
is then rotated continuously about its vertical and horizontal axes to distribute the material uniformly
over the inner surface of the mold (Figure 11.11). The rotating mold then passes through a heated oven.
As the mold is heated, the powdered polymer particles fuse forming a porous skin that subsequently
melts and forms a homogeneous layer of uniform thickness. In the case of a liquid polymer, it flows
over and coats the mold surface and then gels at the appropriate temperature. While still rotating axially,
the mold passes into a cooling chamber where it is cooled by forced air and/or water spray. The mold
is then moved to the work station and opened, and the finished solid part whose outside surfaces and
contour faithfully duplicate those of the inner mold surface is removed. The mold is recharged for the
next cycle.
B. PROCESS VARIABLES
Different types of heating systems, including hot air, molten salts, or circulation of oil through a jacketed
mold have been used. The essential requirement of any heating system is to ensure that the mold is
heated uniformly and at a properly controlled rate so that the desired part thickness is obtained without
causing resin degradation. Given the potential hazards and maintenance problems associated with the
use of molten salts, the use of hot oil has gained wide commercial acceptance because of the relatively
cleaner, cheaper, and safer operation involved.
Figure 11.11 Schematic diagram of rotation molding showing major and minor axes of rotation.
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While the heating cycle has virtually no effect on the properties of the finished part, the cooling rate
determines part shrinkage, final density, brittleness, and other physical properties. Mold cooling is
accomplished by the use of forced air and/or application of water spray.
Rotational molding machines vary from the simple one-arm rotocast system with capability for
producing parts of up to 20 gal size to the industrially predominant three-arm machines that are capable
of producing up to 5000 gal capacity tanks. In the latter case, each arm is always in one of the three
stations: load-unload, oven, or cooling. More recently, shuttle-type machines with molds mounted on
large self-driven shuttle carts have been developed. The carts move on tracks from the oven to the cooling
chamber. These machines can make products of much larger capacities. Rotational molding machines
with microprocessor controls and other solid-state devices for regulating operating variables such cycle
time, oven temperature, rotational speed ratio (ratio of the speed of major to minor axis), and fan and
water on–off times are now available.
The rotational molding operation involves neither high pressure nor shear rates. In addition, precise
metering of materials is not crucial. Therefore, rotational molding machinery is relatively cheap and has
a more extended lifetime. Other advantages of rotational molding include favorable cost–performance
(productivity) ratio, absence of additional finishing operations even of complex parts, minimal scrap
generation, and capability for simultaneous production of multiple parts and colors.
Example 11.8: Explain the variation in the physical properties of rotational molded parts from polypropylene and polystyrene shown in Table E11.8 with changes in the cooling cycle
Table E11.8 Physical Properties of Rotational Molded Parts
Polypropylene
Cooling Time
Polystyrene
Cooling Time
Property
10 min
2 min
10 min
2 min
Specific gravity
Shrinkage
Elongation at break (%)
0.96
0.040
300
0.90
0.015
100
1.20
0.003
1.5
1.19
0.004
1.3
Solution: A reduction in cooling time means faster cooling rates of the rotational molded parts. This
is accompanied by marked changes in the physical properties of the part from polypropylene (crystalline
polymer). On the other hand, the cooling rate has little effect on the part from polystyrene (amorphous
polymer). A longer cooling time permits a greater ordered molecular arrangement in the crystalline
polymer. The resultant enhanced crystallinity leads to higher specific gravity and shrinkage and reduced
brittleness. Cooling rates do not seriously affect molecular arrangement in the amorphous polymer.
VI.
THERMOFORMING4,5
Thermoforming is a process for forming moderately complex shaped parts that cannot be injection
molded because the part is either very large and too expensive or has very thin walls. It consists essentially
of two stages: elevation of the temperature of a thermoplastic sheet material until it is soft and pliable
and forming the material into the desired shape using one of several techniques.
A. PROCESS DESCRIPTION
Thermoforming techniques may be grouped into three broad categories: vacuum, mechanical, and air
blowing processes.
1. Vacuum Forming
The vacuum forming process is shown schematically in Figure 11.12. The plastic sheet is clamped in
place mechanically and heated. A vacuum is then placed beneath the hot elastic sheet, and this makes
atmospheric pressure push the sheet down onto the contours of the cold mold. The plastic material cools
down, and after an appropriate time the cooled part is removed.
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Figure 11.12 Steps in vacuum forming process.
2. Mechanical Forming
In this case, a hot sheet is stretched over a mold or matched molds without the use of air or pressure.
For example, in matched mold forming, the heated sheet is clamped over a female mold or draped over
the mold force (male mold) (Figure 11.13). The two molds are then closed. The resulting part has
excellent dimensional accuracy and good reproduction of the mold detail, including any lettering and
grained surfaces.
3. Air Blowing Process
Here compressed air is used to form the sheet. In one variation, a plastic sheet is heated and sealed
across the female cavity (Figure 11.14). Air at controlled pressure is introduced into the mold cavity.
This blows the sheet upward into an evenly stretched bubble. A plug which fits roughly into the mold
cavity descends on the sheet. When the plug reaches its lowest possible position, a vacuum or, in some
cases, air under pressure is used to complete part formation.
Figure 11.13 Steps in mechanical forming process.
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Figure 11.14 Steps in air-blowing thermoforming process.
From the foregoing discussion, it is evident that the basic steps in the thermoforming process involve
a sequence of operations: loading the sheet or web through the system in increments and heating, forming,
and cooling. Other secondary functions that may be integrated into the process include trimming and
other finishing operations. Thermoforming machinery is categorized on the basis of the arrangement for
performing these operations. In single station machines, the loading, heating, forming, cooling, and
unloading operations are performed in succession. Consequently, even though these machines are versatile, they are characterized by comparatively long cycle times. With in-line machines, which are quite
popular in the packaging industry, the different operations are performed simultaneously in a multistation
in-line system. In this case, therefore, the cycle time is determined by the longest operation in the entire
process. The third category of thermoforming machines is the rotary type, which is supported on a
horizontal circular frame that has three work (and possibly other secondary stations for the loading–unloading, heating, and forming (possibly secondary) operations. By rotating the cut sheets sequentially from station to station, the rotary machine configuration is well suited for high production rates.
Molds for the thermoforming process may be made of wood, metal, or epoxy and are relatively cheap.
They are provided with vents to allow trapped air to escape and release possible pressure buildup.
Temperature control, as we shall see in the following discussion, determines part quality. It is, therefore,
crucial that mold temperature is controlled properly. The mold is consequently provided with channels
for the passage of the cooling liquid.
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B. PROCESS VARIABLES
Thermoplastic materials, in general, can be stretched when hot. However, the degree of success in
forming a part from a hot, stretched sheet material depends on the particular resin, its molecular weight,
and processing conditions like forming temperature and speed. We recall that the thermal response of
thermoplastic polymers depends first on whether the material is amorphous or crystalline and to some
extent on their molecular weights. When heated, crystalline polymers undergo a rather abrupt phase
transformation from the solid to the fluid state, while amorphous polymers undergo a more gradual
transformation. For a given resin, the degree of fluidity depends on molecular weight and the processing
temperature. The higher the molecular weight, the higher the melt strength and consequently the greater
the capacity of the material to be deep-drawn, which is a necessary condition for the proper formation
of intricate parts. On the other hand, materials that are too fluid at the forming temperature are susceptible
to tearing, leading to the production of bad quality parts. Given the influence of forming temperature
on part quality, it is necessary to ensure that sheets have a uniform temperature distribution. This also
calls for a uniform sheet thickness. A variation in sheet thickness causes a nonuniform sheet temperature
distribution resulting in uneven pulling and possible tearing of the sheet. It is also essential to recognize
that variation of shrinkage with sheet orientation can generate forming problems. If there is excessive
differential shrinkage due to sheet orientation, the pull from the clamping frames during forming becomes
unbalanced and the sheet could be pulled out.
VII.
COMPRESSION AND TRANSFER MOLDING4,5
The two most widely used methods for molding thermoset are compression and transfer molding. Thermosets, you will recall, undergo a permanent set, i.e., become essentially insoluble and infusible under the
action of heat. Consequently, techniques for fabricating thermosets must take due cognizance of and make
allowance for the fact that sprues, runners, and gates are not reusable and therefore constitute rejects.
A. COMPRESSION MOLDING
In compression molding, a preweighed amount of material is loaded into the lower half of a heated mold
or cavity. The force plug (plunger) is lowered into the cavity, and pressure, which can range from 20 to
1000 tons, is applied to the powder (Figure 11.15). Under heat and pressure, the powder melts and flows
into all parts of the mold cavity, the resin cross-links thus becoming irreversibly hardened. After an
appropriate time, the mold is opened and the part is ejected while still hot (usually under gravity) and
allowed to cool outside the mold.
The machinery for compression molding is relatively simple, consisting essentially of two platens, which
when brought together, apply heat and pressure to the mold material to form a part of desired shape. The
Figure 11.15
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Schematic of a compression molding operation showing material before and after forming.
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Figure 11.16 Schematic of a transfer molding operation.
platens move vertically, with the cavity usually mounted at the bottom so molding material can be loaded
into it. Material may also be a preform, in which case it has been preheated. Material flow, cure time, and
the ultimate properties of the molded part depend on the mold temperature, which therefore must be
adequately controlled. A number of heating systems are employed for heating the molds. Steam heating
provides uniform mold temperature but is limited to temperatures below 200°C. Electrical heating is both
clean and easy and enjoys wide usage. Circulated hot oil also provides uniform heat. Most thermosets emit
volatile gases or moisture during cure, and escape avenues must be provided for such volatiles to facilitate
processing and obtain good quality parts. This is usually accomplished by opening the mold slightly. With
modern compression molding machines various degrees of automation for feeding the material and ejecting
the molded part are possible. This is achieved by the use of microprocessor controllers.
One of the major advantages of compression molding is that it is relatively inexpensive because of
its simplicity. Also, since there are no sprues, runners, and gates, material waste is reduced considerably.
The consistency of part size is good and the absence of gate and flow marks reduces finishing costs.
B. TRANSFER MOLDING4,5
In transfer molding, (Figure 11.16) a measured charge of preheated thermoset material is placed in a
separate or auxiliary heated chamber called the pot. A plunger is then used to force the molten material
out of the pot through the runner system into the closed mold cavity where curing occurs. As the material
enters the mold, the air from the mold cavity escapes through vents located strategically on the mold.
At the end of the cure cycle, the entire shot, including the gates, runners, sprues, and excess material
remaining in the pot (referred to as cull) is ejected simultaneously with the molded part. If held for too
long in the pot or cylinder, the thermosetting material could cure prematurely into a solid mass.
Consequently, only sufficient material for a single shot is loaded at one time. In addition, preheating the
material is necessary in transfer molding. When cold, the material flows relatively slowly. In this case,
the first material to enter the cavity could cure prematurely resulting in overall improper material mix.
This can result in poor product quality in terms of the surface finish and mechanical properties.
Transfer molding has comparatively shorter cycle and loading times than compression molding. Thick
sections that cure evenly can be molded; however, mold costs are generally higher, and greater volumes
of scrap are generated because of the presence of gates, runners, and sprues.
Examples 11.9: Explain why compression molding of thermoplastics is limited to small quantity
production while screw injection molding is thermoset is also currently used on a limited scale.
Solution: In compression molding of thermoset, once the resin is cured the molded part can be ejected
while still hot and allowed to cool outside the mold. On the other hand, compression molding of
thermoplastics involves heating followed by cooling to solidify the molded part. Therefore, in compression
molding of thermoplastics the relatively longer heat-and-chill cycle times involved are uneconomical
for large volume production.
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UNIT OPERATIONS IN POLYMER PROCESSING
In screw injection molding of thermoplastics, the viscous heat generated assists in melting the resin
material; during processing the resin is always held in the cylinder during and between shots. However,
with thermoset, to avoid resin precure, not only must the barrel and material temperatures be carefully
controlled, the resin cannot be held at high temperature for any length of time before molding.
VIII.
CASTING5
In the casting process, a liquid material is poured into a mold and allowed to solidify by physical (e.g.,
cooling) or chemical (e.g., polymerization) means resulting in a rigid object that generally reproduces
the mold cavity detail with great fidelity. A large number of resins are available and a variety of molds
and casting methods are used in casting processes. Therefore, the choice of liquid material, mold type,
and casting technique is determined by the particular application.
A. PROCESS DESCRIPTION
In casting processes, the resin material is added with an appropriate amount of hardener, catalyst, or
accelerator, mixed manually or mechanically, and then poured into a mold, which is normally coated
with a mold-release agent. Air is removed if necessary and the resin is allowed to solidify. The casting
process is relatively slow and employs comparatively cheap equipment. Molds for casting processes are
fabricated from a wide variety of materials, including wood, plaster and clay, glass, metal, rubber and
latex (for flexible molds), and plastics. To facilitate the removal of the cast part from the mold, moldreleasing agents such as high melting waxes, silicone oils, greases, and some film-forming agents are
used to coat the mold. Among other considerations, the choice of mold-release agents is based upon the
absence of interaction between the resin system and the release agent.
The setting of the resin material is generally exothermic and is usually accompanied by a reduction
in volume. For example, in the casting of acrylic, the amount of heat evolved is about 13.8 kcal/mol,
while the volume reduction can be as large as 21%. The quantity of heat liberated depends on the size,
but is independent of the shape of the casting. However, the rate of heat dissipation depends both on
the size and shape of the casting.
Therefore, different methods have to be employed in casting thick or thin sections. Thin sections can
be cast at room temperature with minimal possibility of cracking of the casting since heat can be
dissipated rapidly. On the other hand, with thick sections, particularly those whose shapes limit the rate
of heat removal, large temperature gradients exist between the interior and exterior sections of the casting.
This generates internal stresses and enhances the possibility of cracking of the cast part. In this case,
therefore, where the quantity of heat liberated cannot be changed, the rate of heat dissipation is controlled
by carefully selecting the hardener and a mold material with appropriate thermal properties and by using
a possible stepwise increase in casting temperature. To reduce shrinkage and the attendant built-in stresses
in the cast part, flexibilizers, diluents, flexibilized resins or hardeners, or fillers are used.
B. CASTING PROCESSES
1. Casting of Acrylics
Acrylic castings are prepared with polymers derived from methacrylic ester monomers. They usually
consist of poly(methyl methacrylate) or its copolymers as the major component modified with small
amounts of other monomers. Acrylic castings are made by heating the monomers or partially polymerized
syrups containing 0.02 to 0.1% radical initiator (e.g., peroxide or azo-bis-isobutyronitrile) in a mold.
Syrups are prepared by dissolving finely divided polymer in the monomer. As indicated earlier, the
polymerization reaction involved in the acrylic casting process is exothermic and is usually accompanied
by about 15 to 21% volume shrinkage. Because of the exothermic nature of this reaction, measures are
usually taken to deal with the problems of heat removal and residual stresses in the casting.
The outstanding optical properties of acrylic casting make acrylic sheets invaluable in applications
where excellent transparency and resistance to UV are imperative. The fabrication of acrylic sheets
therefore predominates in the acrylic casting process. Cast acrylic resins also account for most of the
embodiments used for decorative or study purposes. In this case, hard polymers of ethyl or methyl
methacrylate are used. Rods and tubes are also prepared from acrylic castings.
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POLYMER SCIENCE AND TECHNOLOGY
2. Casting of Nylon
The nylon casting process consists basically of four steps. These are the melting of the monomer, which
is usually lactam flakes, the adding of the catalyst and activator, the mixing of the melts, and the casting
process itself. Cocatalyzed anionic polymerization is currently the most widely used nylon casting
method. The cocatalysts are strong bases and their salts with imides and lactams.
Since absorbed water can cause catalyst decomposition and hence incomplete polymerization and
since lactam monomer flakes are highly hygroscopic, the melting of monomer is carried out under
appropriately controlled temperature and humidity conditions. All additives are also completely dried
and then mixed with the monomer in stainless steel vessels while flushing with inert gas under thermostatically controlled temperature. Molds can be of the single type fabricated from silicone rubber, epoxy,
or sheet steel or the more expensive tool steel used in tight tolerance cast-to-size parts casting.
The nylon casting process is relatively more economical and is a more practical technique for the
production of large and thick parts than comparable extrusion and injection molding processes. In
addition, the crystallinity and molecular weight of cast nylon are higher than those of extruded or molded
nylon. Consequently, cast nylon has a much higher modulus and heat deflection temperature, improved
solvent resistance, and better hygroscopic characteristics and dimensional stability.
IX.
PROBLEMS
11.1. A small business entrepreneur is considering buying an extruder for a 2-in. (ID) PVC pipe production.
A two 8-h/d shift operation for a 300-day work year is planned. If the extruder is to be operated to
produce 20 ft of pipe per hour and pipe thickness is 0.5 in., suggest the power rating for the electric
motor appropriate for this extruder. PVC has a density of 1.44 g/cm3.
11.2. Explain the following observations.
a. When complicated shapes are to be extruded, they are usually made from amorphous polymers rather
than crystalline polymer.
b. Under comparable thermal conditions, extrusion rate is generally higher with amorphous polymers
than crystalline polymers.
c. Many extruders are now equipped with dehumidified hopper driers.
11.3. The following table shows some thermal properties of three thermoplastic polymers. Compute the screw
output for each polymer material if the screw drive power input is 60 hp and the mechanical efficiency
is 75%. Assume that the heat requirement for molding the materials is satisfied by the viscous dissipation
and feed temperature is 77°F.
Polymer
Polystyrene
Polyethylene
Nylon 6,6
Avg Molding
Temp. (°F)
Specific
Heat (Btu/lb °F)
Heat of
Fusion (Btu/lb)
500
440
530
0.45
0.91
0.63
0
104
56
11.4. For a 2.5-in.-diameter screw at 200 rpm the maximum permissible drive input power is 40 hp. What is
the maximum permissible drive input power for a 4.5-in.-diameter screw at (a) 150 rpm; (b) 200 rpm.
11.5. A syringe 6 in. long, 1 in. in diameter, and 0.1 in. thick is to be injection molded. The stress generated
in the mold cavity and core material as the mold is filled with the plastic melt is 180,000 psi. Estimate
the change in volume of the syringe if the mold cavity and core materials are made of:
a. Steel
b. Copper
c. Aluminum
Assume that the volume change is due essentially to the change in the core diameter.
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UNIT OPERATIONS IN POLYMER PROCESSING
307
11.6. For crystalline polymers, high mold temperatures result in enhanced tensile strength but reduced clarity
of the molded part. Explain.
11.7. Decide which of the two processes (continuous extrusion blow molding or intermittent extrusion blow
molding) is more suitable for the production of bottles from:
a. Polyethylene
b. Poly(vinyl chloride)
Explain the basis of your decision.
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
Richardson, P.N., Introduction to Extrusion, Society of Plastics Engineers, CT, 1974.
Weir, C.L., Introduction to Molding, Society of Plastics Engineers, CT, 1975.
Bernhardt, Ed., Processing of Thermoplastic Materials, Robert E. Krieger, New York, 1974.
Kaufman, H.S. and Falcetta, J.J., Eds., Introduction to Polymer Science and Technology, John Wiley & Sons, New
York, 1977.
1989/90 Modern Plastics Encyclopedia, McGraw-Hill, New York, 1990.
Frados, J., Ed., The Story of the Plastics Industry, 13th ed., Society of the Plastics Industry, New York, 1977.
Chastain, C.E., A plastics primer, in 1969/70 Modern Plastics Encyclopedia, McGraw-Hill, New York, 1970.
Dunham, R.E., Plast. Eng., 48(8), 21, 1992.
Mock, J., Plast. Eng., 39(12), 17, 1983.
Copyright 2000 by CRC Press LLC
Chapter 12
Solution Properties of Polymers
I.
INTRODUCTION
In order to gain insight into the polymer dissolution process, let us briefly review the dissolution of lowmolecular-weight (simple) substances. We know, for example, that while oil will not mix with water, an
oil stain in clothing can be removed rather easily by using hydrocarbon solvents like naphtha. On the
other hand, ordinary table salt, or sodium chloride, dissolves readily in water but not in gasoline. As
will become evident presently, the physical phenomena associated with the solubilities of various
substances in different solvents are intimately tied with the nature of the solutes and solvents. For
example, in molecular crystals, the attractive forces are of the dipole–dipole or London dispersion type,
which is relatively weak and therefore fairly easy to break apart. Consequently, this type of solid dissolves
to an appreciable degree in nonpolar solvents, where the molecules are held together by London-type
attractive forces also. However, crystals will not dissolve to any great extent in polar solvents since the
strong attraction between the polar solvent molecules cannot be overcome by the much weaker solute–solvent interaction forces. By similar arguments, polar solutes and ionic solids are soluble only in
polar solvents. They are insoluble in nonpolar solvents because the weak solute–solvent interaction is
not strong enough to overcome the strong attractive forces between the solute molecules and hold them
apart. In essence, therefore, when a solute dissolves in a solvent, solute–solute molecular contacts are
replaced by solute–solvent contacts. Consequently, for solute particles to enter into solution, the solute–solvent forces of attraction must be sufficient to overcome the forces that hold the solid together.
It follows from the above discussion that polymers, by virtue of their macromolecular nature, will
be soluble only in selected solvents. The polymer solution process is certainly more complex than that
of simple compounds. The dissolution of both simple compounds and polymers depends on the nature
of the solute and solvent, but in addition the dissolution of polymers is affected by the viscosity of the
medium, polymer texture, and molecular weight. Dissolution of a polymer is necessarily slow and is a
two-staged process: first, the solvent molecules diffuse into the polymer producing a swollen gel; second,
the gel breaks down slowly forming a true solution. In some cases and depending on the nature of the
polymer, only the first step occurs. However, if the polymer–polymer interaction forces can be overcome
by polymer–solvent attraction, then the second stage will follow, albeit slowly. For example, unvulcanized
rubber will dissolve in solvents in which vulcanized rubber will only swell. In other cases, materials
with strong polymer–polymer intermolecular forces due to, say, cross-linking (phenolics), crystallinity
(Teflon), or strong hydrogen bonding (native cellulose) will not dissolve in any solvent at ordinary
temperatures and will exhibit only a limited degree of swelling.
II.
SOLUBILITY PARAMETER (COHESIVE ENERGY DENSITY)
From thermodynamic considerations, it is possible to predict whether or not a given solute will be soluble
in a given solvent using the relation:
∆G m = ∆H m − T∆Sm
(12.1)
where ∆Gm, ∆Hm, and ∆Sm are free energy, heat, and entropy of mixing, respectively. Solubility will
occur if the free energy of mixing ∆Gm is negative. The entropy of mixing is believed to be always
negative. Therefore, the sign and magnitude of ∆Hm determine the sign of ∆Gm. If we consider an ideal
solution of two small spherical molecules with identical size and intermolecular forces, molecules of
one type can replace neighbors with molecules of another type without changing the total energy of the
system. This interchangeability of neighbors is the source of the configurational entropy term or the
entropy of mixing ∆Sm. Since we are dealing with ideal conditions, the heat of mixing ∆Hm is zero
0-8493-????-?/97/$0.00+$.50
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POLYMER SCIENCE AND TECHNOLOGY
because the two types of molecules have the same force fields, and consequently, ∆Gm = –T∆Sm. However,
departure from ideality normally occurs because the intermolecular forces operative between similar and
dissimilar molecules give rise to a finite heat of mixing. In this case, the energy of mixing associated
with the formation of contact between two dissimilar molecules can be shown to be positive. Therefore,
mixing is generally endothermic for nonpolar molecules in the absence of strong intermolecular attraction
such as hydrogen bonding.
Using similar arguments, Hildebrand and Scott1 showed that
[(
∆H m = Vφ, φ2 ∆E1v V1
)
12
(
− ∆E 2v V2
)
]
12 2
(12.2)
where V, V1, V2, are the volumes of the solution and components and the subscripts 1 and 2 denote the
solvent and polymer, respectively. ∆Ev is the molar energy of vaporization and φ1 and φ2 are volume
fractions. In terms of the heat of mixing per unit volume, Equation 12.2 becomes
[(
∆H m
= φ, φ2 ∆E1v V1
V
)
12
(
− ∆E 2v V2
)
]
12 2
(12.3)
The quantity ∆E/V is referred to as the cohesive energy density (CED): its square root is the solubility
parameter (δ). Thus
CED =
∆E
= δ2
V
(12.4)
Equation 12.3 may be rewritten:
∆H m
= φ, φ2 δ1 − δ 2
V
[
]
2
(12.5)
To a first approximation and in the absence of strong intermolecular forces like hydrogen bonding, a
polymer is expected to be soluble in a solvent if δ1 – δ2 is less than 1.7–2.0. Equation 12.5 is valid only
when ∆Hm is zero or greater. It is invalid for exothermic mixing, that is, when ∆Hm is negative. Typical
values of δ for various types of solvents are shown in Table 12.1. Values for some polymers were listed
in Table 3.7. The magnitude of the enthalpy of mixing can be conveniently estimated from these tables.
Table 12.1 Values for Different Solvents
Solvent
δ
Poorly hydrogen bonded
n-Pentane
7.0
n-Heptane
7.4
Apco thinner
7.8
Solvesso 150
8.5
Toluene
8.9
Tetrahydronaphthalene
9.5
O-Dichlorobenzene
10.0
1-Bromonaphthalene
10.6
Nitroethane
11.1
Acetonitrile
11.8
Nitromethane
12.7
Solvent
δ
Moderately hydrogen bonded
Diethyl ether
7.4
Diisobutyl ketone
7.8
n-Butyl acetate
8.5
Methyl propionate
8.9
Dibutyl phthalate
9.3
Dioxane
9.9
Dimethyl phthalate
10.7
2,3-Butylene carbonate
12.1
Propylene carbonate
13.3
Ethylene carbonate
14.7
Solvent
δ
Strongly hydrogen bonded
2-Ethylhexanol
9.5
Methyl isobutyl carbinol
10.0
2-Ethylbutanol
10.5
n-Pentanol
10.9
n-Butanol
11.4
n-Propanol
11.9
Ethanol
12.7
Methanol
14.5
From Burrel, H. and Immergut B., in Polymer Handbook, Brandrup, J. and Immergut E.M., Eds., John Wiley &
Sons, New York, 1967. With permission.
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SOLUTION PROPERTIES OF POLYMERS
Table 12.2 Molar Attraction Constants, E (cal cm3)/mol
Group
–CH3
–CH2–
>CH–
>C<
CH2›
–CH›
>C›
–CH› aromatic
–C› aromatic
–O– ether, acetal
–O– epoxide
–COO–
>C–O
–CHO
(CO)2O
–OH–
OH aromatic
–H acidic dimer
E
Group
E
148
131.5
86
32
126.5
121.5
84.5
117
98
115
176
326.5
263
293
567
226
171
–50.5
NH2
–NH–
–N–
C› N
NCO
–S–
Cl2
Cl primary
Cl secondary
Cl aromatic
F
Conjugation
cis
trans
six-membered ring
ortho
meta
para
226.5
180
61
354.5
358.5
209.5
342.5
205
208
161
41
23
–7
–13.5
–23.5
9.5
6.5
40
From Hoy, K.L., J. Paint Technol., 42, 76, 1970. With permission.
In addition, the solubility parameter can be estimated from the molar attraction constants, E, using
the structural formula of the compound and its density (Table 12.2). For a polymer:
δ2 =
ρΣ E
M
(12.6)
where ρ and M are the density and molecular weight, respectively, of the polymer repeating unit.
Example 12.1: Estimate the solubility parameters of the following polymers:
a.
b.
c.
d.
Low-density polyethylene (LDPE)
High-density polyethylene (HDPE)
Polypropylene (PP)
Polystyrene (PS)
Solution: From Equation 12.6,
Copyright 2000 by CRC Press LLC
Polymer
Repeating Unit
M
ρ
∑E
δ
a. LDPE
–CH2–CH2–
28
0.92
0.92 [131.5 + 131.5]
28
8.6
b. HDPE
–CH2–CH2–
28
0.95
0.93[131.5 + 131.3]
28
8.9
c. PP
–CH2–CH–
|
CH3
42
0.90
0.90 [131.5 + 86 + 148]
42
7.8
d. PS
–CH2–CH–
|
104
1.04
1.04 [131.5 + 86 + (5 × 117) + 98]
104
9.0
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POLYMER SCIENCE AND TECHNOLOGY
Figure 12.1 A segment of a polymer chain, showing four successive chain atoms. The first three of these
define a plane, and the fourth can lie anywhere on the indicated circle perpendicular to and bisected by the plane.
Figure 12.2 Fully extended chain with every carbon atom in trans location and in the same plane.
III.
CONFORMATIONS OF POLYMER CHAINS IN SOLUTION
Molecules in the dissolved, molten, amorphous, and glassy states of macromolecules exist as random
coils. This is a result of the relative freedom of rotation associated with the chain bonds of most polymers
and the myriad number of conformations that a polymer molecule can adopt. As a consequence of the
random coil conformation, the volume of a polymer molecule in solution is many times that of its
segments alone. The size of the dissolved polymer molecule depends quite strongly on the degree of
polymer–solvent contact. In a thermodynamically good solvent, a high degree of interaction exists
between the polymer molecule and the solvent. Consequently, the molecular coils are relatively extended.
On the other hand, in a poor solvent the coils are more contracted. Many properties of macromolecules
are dictated by the random coil nature of the molecules. We now discuss briefly the conformational
properties of polymer chains.
First, we need to develop a realistic physical picture of a polymer molecule. To do this, let us consider
the properties of a single molecule in a dilute solution isolated from its neighbors by the molecules of
the solvent. Let us consider initially a short segment of this molecule consisting only of four methylene
groups, as shown in Figure 12.1. We define a plane by the first three carbon atoms C1 to C3. Since there
is free rotation about the C–C bond, the fourth carbon atom, C4, can be found in any position on the
circle shown in the figure. Of course, some positions are more probable than others since absolutely
free rotation about bonds is precluded by steric hindrance. Each successive atom on the chain can, in
turn, occupy any random position on similar cycles based on the position of the preceding atom. It is
easy to visualize, therefore, that for a molecule composed of thousands of atoms, the number of possible
conformations is virtually limitless. One of these conformations is the fully extended chain, in which
each successive carbon atom is coplanar and translocated with respect to the earlier atoms in the chain.
The conventional formula for polymethylene expressed this configuration (Figure 12.2).
A. END-TO-END DIMENSIONS
Any physical property of a polymer molecule that depends on its conformation can ordinarily be
expressed as a function of some sort of average dimension. The polymer dimension that is most often
used to describe its spatial character is the displacement length, which is the distance from one end of
the molecule to the other. For the fully extended chain, this quantity is referred to as the contour length.
Given the extremely large number of possible conformations and number of chains, a statistical average,
such as the root-mean-square end-to-end distance, (r2)1/2, is required to appropriately express this quantity.
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SOLUTION PROPERTIES OF POLYMERS
Figure 12.3 Freely jointed model of a polymer molecule with fixed and equal bond length and unrestricted
value of bond angle.
Figure 12.4 A highly schematic representation of a random-coil polymer chain, with one end at the origin of
a coordinate system and the other in a volume element dx dy dz at a distance (x2 + y2 + z2)1/2 from the origin.
Another way of expressing the effective size of a molecule is the radius of gyration, (S2)1/2. This is the
root-mean-square distance of the elements of the chain from its center of gravity. For linear polymers,
the root-mean-square end-to-end distance has a simple relation with its radius of gyration (S2)1/2, given
by Equation 12.7:
r 2 = 6 s2
(12.7)
B. THE FREELY JOINTED CHAIN
We begin by considering first a hypothetical freely jointed chain consisting of n bonds of fixed length l,
jointed in a linear sequence without any restriction on the magnitude of the bond angles. Since the bond
angles are free to assume all values with equal probability and rotations about bonds are similarly free,
a given bond can assume all directions with equal probability regardless of the directions of its neighbors
in the chain. Such a chain is illustrated in Figure 12.3. We are aware, however, that no real polymer
approximates this model. The problem of determining the end-to-end distance, r, is reduced to that of
random flight that occurs in diffusion theory. The question is determining the probability of finding one
end of the chain in a volume element dx dy dz at a distance r from the other end (Figure 12.4). It can
be shown that the solution of this random flight problem, for a very long chain unperturbed by selfinteractions of long-range and external constraints, the probability per unit volume W(r), is a Gaussian
distribution function shown in Figure 12.5. This shows that there is a much greater chance of finding
the two ends close to each other and that as the two ends move farther apart, the probability decreases
continuously. Another way of interpreting the curve is in terms of density distribution of chain ends.
That is, if one end of the chain is located at the origin, the probability of finding the other end in a unit
volume close to the origin is highest. On the other hand, granted that one end of the chain is at the
origin, we want to find out the probability that the other end of the chain is in a spherical shell of
thickness dr and at a distance r (Figure 12.6). This is given by Equation 12.8:
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POLYMER SCIENCE AND TECHNOLOGY
Figure 12.5 The probability W(x, y, z) of finding the end of the chain of Figure 12.4 in the volume element dx
dy dz as a function of r (in angstrom units) calculated for a chain of 104 links, each 2.5 Å long.
Figure 12.6 Spherical shell thickness dr at distance r from origin.
∞
∫ W(r) 4πr dr = 1
2
(12.8)
0
The integral is equal to one since there is a definite chance of finding the desired end in space. However,
since the chain length has a finite value, the upper limit of the integral should appropriately be the
contour length. That is,
∞
∫ W(r) 4πr dr = r
2
2
(12.9)
0
This is shown in Figure 12.7, which demonstrates that the maximum probability corresponds to the most
probable dimension for the chain. Assuming the root-mean-square end-to-end distance represents the
most probable chain dimension, then, according to random flight theory,
(r )
2
f
12
= ln1 2
(12.10)
Here r2 is the square of the magnitude of the end-to-end distance averaged over all conformations, and
f denotes the result of random flight calculation.
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SOLUTION PROPERTIES OF POLYMERS
Figure 12.7 The probability of W(r) of finding the end of the chain of Figure 12.4 in a spherical shell of thickness
dr at a distance r from the origin.
C. REAL POLYMER CHAINS
The freely jointed model discussed in the previous section grossly underestimates the true dimensions
of real polymer molecules because it ignores restrictions to completely free rotation arising from fixed
bond angles and steric effects (short-range interactions). Also, it fails to account for the effects of longrange interactions that result from the inability of two chain segments to physically occupy the same
space at the same time.
1. Fixed Bond Angle (Freely Rotating)
In real polymer chains, the direction assumed by a given bond depends strongly on that of its immediate
predecessor and to a smaller extent on the orientation of nearby bonds. While the structure of the chain
unit determines the ultimate nature of the restrictions on given a bond, the overall effect of these shortrange interactions is to expand the conformation of the real polymer chain relative to that obtained from
the random flight model of the same contour length. The effect of fixed bond angles is a modification
of the expression for the unrestricted bond angles from random flight (Equation 12.11):
1 − cos θ
r 2 = nl 2
1 + cos θ
(12.11)
where θ is the bond angle.
2. Fixed Bond Angles (Restricted Rotation)
Restriction to free rotation about bonds due to steric interferences between successive units of the chain
leads to a further expansion of chain dimensions. Let us amplify this by considering Figure 12.8. Here
θi is the valence (bond) angle between bonds i and i + 1; αi is its supplement (i.e., αi = 180 – θi) and
φi is the angle of rotation about i. It is a measurement of the dihedral angle between the planes defined
by the two planes i – 1, i; and i + 1. The direction of bond i + 1 is, as we have seen from the above
discussion, a function of bond angle θi. In addition, it also depends on the direction of bond i – 1 through
the angle of rotation φi. However, as a result of hindrance to free rotation, φi cannot assume all values
from 0 to 2π with equal probability; it is limited to certain preferred values. The same argument holds
for each bond in relation to its predecessor. When some conformations are preferred over others as a
result of restriction to free rotation, Equation 12.11 becomes
Figure 12.8 Rotation about bond i.
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POLYMER SCIENCE AND TECHNOLOGY
1 − cos θ 1 + cos φ
r 2 = nl 2
1 + cos θ 1 − cos φ
(12.12)
The dimension obtained from random flight calculation, which includes the effects of bond angles
and hindrances to rotation about bonds, is referred to as the unperturbed dimension of the polymer chain.
It is represented by the symbol (r02)1/2. The subscript zero is used to emphasize the condition that the
molecule is subject only to local constraints involving the geometrical character of the bond structure
and restricted rotation.
3. Long-Range Interactions
The freely jointed model assumes implicitly that two elements of the same molecule, possibly remote
along the chain, can occupy the same position in space at the same time. In real polymer chains
conformations in which this exists are impossible. Each segment of a real polymer chain exists within
a volume that excludes all other segments. The number of such forbidden conformations that must be
excluded is greater for the more compact arrangements with smaller values of r2. The net effect of such
long-range interaction is to expand the actual chain dimension (r2)1/2 over its unperturbed dimensions,
(r02)1/2 by an expansion coefficient defined by Equation 12.13:
(r )
2
12
( )
= α ro2
12
(12.13)
The magnitude of α depends on the environment of the polymer molecule. In a thermodynamically
good solvent, where there is strong polymer–solvent interaction, α is large. By the same token, α is
small in a poor solvent. The value of α is therefore an indirect measure of the magnitude of the
polymer–solvent interaction forces, or the solvent power. When α is 1, these forces become zero and
by definition the polymer assumes its unperturbed conformation. However, polymer–solvent interaction
forces and, consequently α, depend on temperature. For a given solvent, the temperature at which α = 1
is referred to as Flory temperature θ. When a solvent is used at T = θ, it is called theta (θ) solvent.
Alpha (α) increases with n (or M) with solvent power and with temperature. Other parameters for
characterizing the dimensions of polymer molecules are summarized in Table 12.3. The physical significance of the parameters is illustrated by a brief discussion of the Flory characteristic ratio and the
Stockmayer–Kurato ratio.
a. Flory’s Characteristic Ratio (C∞ )
The quantity α represents the effect of “long-range interactions.” It describes the osmotic swelling of
the chain due to polymer–solvent interaction. On the other hand, r02 represents the effect of “short-range
Table 12.3 Parameters Characterizing Chain Dimensions
Parameter
Flory’s characteristic ratio (C∞)
Relation to Unperturbed Dimension
C∞
=
r02
nl 2
(r )
2
0
12
Stockmayer–Kurato ratio (σ)
σ=
Characteristic length (a)
a = C1∞2 l j1 2
(r )
2 12
0f
j = number of backbone bonds
per monomer unit (usually 2)
Kratky–Porod persistence length (ap)
Copyright 2000 by CRC Press LLC
ap =
(
)
l
C +1
2 ∞
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SOLUTION PROPERTIES OF POLYMERS
Table 12.4 C∞ and σ for Some Polymers
Polymer
Solvent
Temperature (°C)
C∞
σ
Polyethylene
Polypropylene
Isotactic
Syndiotactic
Atactic
Poly(methylmethacrylate)
Polystyrene
Isotactic
Atactic
Decalin
140
6.8
1.84
Tetralin
Heptane
Decalin
Benzene
140
30
135
21
5.2
6.1
5.3
9.0
1.61
1.75
1.63
2.12
Benzene
Cyclohexane
30
34
10.5
10.4
2.30
2.28
From Kurata, M., Tsunashima, Y., Iwama, M., and Kamada, K., in Polymer Handbook,
2nd ed., Brandrup, J. and Immergut, E.H., Eds., John Wiley & Sons, New York, 1975.
With permission.
interactions” induced by bond angle restrictions and steric hindrances to internal rotation. Flory’s
characteristic ratio, C∞, is defined by
C∞ =
r02
nl 2
(12.14)
It is a measure of the effect of short-range interactions.
σ)
b. Stockmayer–Kurato Ratio (σ
The freely rotating state is a hypothetical state of the chain in which the bond angle restrictions are
operative but in which there are no steric hindrances to internal rotation. The Stockmayer–Kurato ratio,
α, reflects such rotational isomerism preferences. That is, it is a measure of the effect of steric hindrance
to the average chain dimension. It is given by
(r )
σ=
(r )
2
o
2
0f
12
12
(12.15)
Here (r0f2)1/2 is the root-mean-square end-to-end distance of the hypothetical chain with the same bond
angles but with free rotation around valence cones. Table 12.4 lists the values of C∞ and α for some
polymers.
Example 12.2: A polyethylene molecule has a degree of polymerization of 2000. Calculate (a) the total
length of the chain and (b) the contour length of the planar zigzag if the bond length and valence angle
are 1.54 Å and 110°, respectively.
A
54
1.
°
(Str. 1)
110°
r
Solution:
a. The total length of the chain, L, is the sum of the length of each bond, l. It is the total distance
traversed going from one end of the chain to the other following the bonds.
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POLYMER SCIENCE AND TECHNOLOGY
L = nl
where n = the number of bonds. Each monomer contributes the equivalent of 2 C–C bonds.
Therefore,
n = 2 × DP
= 2 × 2000 = 4000
L = 1.54 Å × 4000 = 6.16 × 103 Å
110
1.
54
A
°
b. The contour length is that of the fully extended chain conformation.
(Str. 2)
35
r =
=
=
=
n (1.54 cas 35)
2 × DP (1.54 cas 35)
2 × 2000 × 1.54 cas 35
5.05 × 103 A8
IV.
THERMODYNAMICS OF POLYMER SOLUTIONS
As may be expected, polymers behave differently toward solvents than do low-molecular-weight compounds. Studies of the solution properties of polymers provide useful information about the size and
shape of polymer molecules. In this section we discuss how some of the molecular parameters discussed
in the previous sections are related to and can be calculated from thermodynamic quantities. We start
with a discussion of the simplest case of an ideal solution. This is followed by a treatment of deviations
from ideal behavior.
A. IDEAL SOLUTION
Consider a binary mixture of two types of molecules that are roughly identical in size, shape, and external
force field. Such a mixture constitutes an ideal solution. Thus, one of the components of an ideal solution
may replace another without seriously disturbing the circumstances of immediate neighbors in the
solution. Raoult’s law provides an appropriate basis for the treatment of an ideal solution. Raoult’s law
states that the activity, a, of a solvent in the solution is equal to its mole fraction n1:
a1 =
N1
= n1
N1 + N 2
(12.16)
where N1, N2 are the number of solvent and solute molecules, respectively. The free energy of mixing
∆Gmix is given by Equation 12.17:
∆G mix = ∆H mix − T ∆ Smix
(12.17)
For the ideal solution, since the intermolecular force fields around the two types of molecules are the
same, ∆Hmix = 0. Thus Equation 12.17 becomes
∆G mix = − T ∆ Smix
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(12.18)
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SOLUTION PROPERTIES OF POLYMERS
The entropy of an ideal solution is greater than that of the pure components because the number of
possible arrangements of the molecules of the components of a solution is much greater than that for
the molecules of a pure components. If N0 is total number of molecules (i.e., N0 = N1 + N2), then the
total number of possible combinations of N0 taken N1 at a time is
W=
N 0!
N1! N 2!
(12.19)
From the Boltzmann relation,
∆Smix = k ln W
= k ln
(12.20)
N 0!
N1! N 2!
∆Smix = k ln (N1 + N 2 )! − k ln N1! − k ln N 2!
(12.21)
Using Stirling’s approximation, ln N! = N ln N – N, and rearranging in terms of mole fractions,
Equation 12.20 becomes
[
∆Smix = k N1 ln n1 + N 2 ln n 2
]
(12.22)
According to Raoult’s law, the partial vapor pressure of each component in a mixture is proportional
to its mole fraction. Thus for a binary solution consisting of solvent and a polymer, the partial pressure
of the solvent in the solution, P1, is related to that of the pure solvent P10 by
P1 = n1 P10
(12.23)
Since the molecular weight of a polymer is usually at least three orders of magnitude greater than that
of the solvent, for a small weight fraction of the solvent N1 @N2, consequently the mole fraction of the
solvent approaches unity very rapidly. This, in effect, means that following Raoult’s law, the partial
pressure of the solvent in the solution should be virtually equal to that of the pure solvent over most of
the composition range. Available experimental data do not confirm this expectation even if volume
fraction is substituted for mole fraction. Polymer solutions exhibit large deviations from the ideal law
except at extreme dilutions, where ideal behavior is approached as an asymptotic limit.
B. LIQUID LATTICE THEORY (FLORY–HUGGINS THEORY)
One of the reasons for the failure of the ideal solution law is the assumption that a large polymeric
solute molecule is interchangeable with the smaller solvent molecule. The law also neglects intermolecular forces since the heat of mixing (∆Hmix) is assumed to be zero. The Flory–Huggins theory attempted
to remedy these shortcomings in the ideal solution law.5,6–8
1. Entropy of Mixing
In order to calculate the entropy of mixing of a polymer solution, the polymer chain is assumed to be
composed of x chain segments, where x is the ratio of the molar volumes of the solute polymer and the
solvent. Each chain segment represents the portion of the polymer molecule equal in size to a solvent
molecule. This means that a polymer chain segment can replace a solvent molecule in the liquid lattice
and vice versa. However, unlike a solution containing an equal proportion of a monomeric solute, a
polymer solution requires a set of x contiguous or consecutive lattice cells to accommodate the polymer
molecule (Figure 12.9). A further assumption is that the solution is sufficiently concentrated that the
occupied lattice sites are randomly distributed instead of being sparse and widely separated, which would
exist in a very dilute solution.
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POLYMER SCIENCE AND TECHNOLOGY
Figure 12.9 Binary solution of a monomer showing distribution of solute in lattice cells (a) compared with that
of a polymer, which requires a set of contiguous cells to accommodate the solute (b).
The entropy of mixing of a polymer solution is relatively smaller than that of an equivalent proportion
of a monomeric solute. This is because the macromolecular nature of the polymer molecule severely
restricts the number of possible arrangements of polymer segments in the lattice sites. Once a given
segment occupies a site, the number of sites available for adjacent segments becomes seriously limited.
The dissolution of the polymer is conceived to occur in two consecutive steps: first, the polymer is
disoriented, and then the disoriented polymer mixes with the solvent. The entropy of mixing of the
disoriented polymer and solvent has been shown by Flory to be given by
[
∆Smix = k N1 ln ν1 + N 2 ln ν2
]
(12.24)
where the subscripts 1 and 2 denote the solvent and polymer, respectively, and ν1 and ν2 are their volume
fractions defined as
ν1 =
N1
N1 + xN 2
(12.25)
ν2 =
xN 2
N1 + xN 2
(12.26)
2. Heat and Free Energy of Mixing
To derive an expression for the heat of mixing of a polymer solution, the pure solvent and pure liquid
polymer are taken as reference states. The heat of mixing is considered to be the difference between the
total interaction energy in the solution relative to that of the pure components. It arises from the
replacement of some of the solvent–solvent and polymer–polymer contacts in the pure components with
solvent–polymer contacts in the solutions. As the distance between uncharged molecules increases, the
forces between them decrease very rapidly. Consequently, interactions between elements that are not
immediate neighbors can be safely neglected. If only the energies developed by first-neighbor elements
are considered, then the heat of mixing of polymer solution, like that of ordinary solutions, is given by
∆H mix = χ1 kTN1 ν2
(12.27)
where χ characterizes the interaction energy per solvent molecule divided by kT. Combining
Equation 12.24 with that of the configurational entropy (Equation 12.27) gives the Flory–Huggins expression for the free energy of mixing of a polymer solution:
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321
SOLUTION PROPERTIES OF POLYMERS
∆G mix = kT (N1 ln ν1 + N 2 ln ν2 + χ1 N1 ν2 )
(12.28)
Quantities that are determinable from experiments can be derived from Equation 12.28. For example,
by differentiating the expression with respect to N1, the number of solvent molecules, and multiplying
the result by Avogadro’s number, the relative partial molar free energy ∆G1 is obtained:
[
∆G i = RT ln (1 − ν2 ) + (1 − 1 x) ν2 + χ1 ν22
]
(12.29)
We note that ∆G1 is expressed on a per mole basis. The activity of the solvent, a1, and the osmotic
pressure of the solution, π, are given by Equations 12.30 and 12.31, respectively:
ln a1 = ln (1 − ν2 ) + (1 − 1 x) ν2 + χ1 ν22
π=−
[
RT
ln (1 − ν2 ) + (1 − 1 x) ν2 + χ1 ν22
V1
(12.30)
]
(12.31)
where V1 is the molar volume of the solvent. Expanding the logarithmic term and neglecting higher
order terms, the expression for the osmotic pressure becomes
π=
RT ν2 1
1
+
− χ1 ν22 + ν23 + L
V1 x
2
3
(12.32)
The above thermodynamic expressions for a binary solution of a polymer in a solvent include the
dimensionless parameter χ1. Its value can be determined by measuring any of the experimentally
obtainable quantities, like solvent activity or the osmotic pressure of the solution. The constancy of χ1
over a wide composition range would be a confirmation of the validity of the Flory–Huggins theory.
Figure 12.10 represents such a plot obtained by the measurement of solvent activities for various systems.
Only in the case of the nonpolar rubber–benzene system was the predicted constancy of χ1 observed;
other systems showed marked deviations from theory.
Equation 12.29 can be separated into contributions from the heat of dilution and configurational
entropy, as shown in Equations 12.33 and 12.34, respectively:
∆H1 = RT χ1 ν22
[
∆S1 = − R ln (1 − ν2 ) + (1 − 1 x) ν2
(12.33)
]
(12.34)
where ∆H1, is the relative partial molar heat content while ∆S1 is the relative partial molar configurational
entropy of the solvent in solution. ∆H1 was determined from the heat of mixing obtained from calorimetric
methods. The predicted concentration dependence of ∆H1 was also not observed.
C. DILUTE POLYMER SOLUTIONS (FLORY–KRIGBAUM THEORY)
The Flory–Huggins lattice model assumed a uniform density of lattice occupation. This assumption
holds only for concentrated solutions; it is invalid for dilute polymer solutions. According to the
Flory–Krigbaum model, a very dilute polymer solution consists of loose domains or clusters of polymer
chain segments separated by intervening regions of pure solvent. Each such cloud is assumed to be
approximately spherical with an average density that is maximum at the center and that decreases with
increasing distance from the center in an approximately Gaussian function. Each molecule within a
domain or occupied volume tends to exclude all other molecules. Long-range thermodynamic interactions
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POLYMER SCIENCE AND TECHNOLOGY
Figure 12.10 Experimentally observed variation of χ1 with concentrations: (a) poly(dimethylsiloxane) in benzene; (b) polystyrene in benzene; (c) rubber in benzene; curve (d) polystyrene in toluene. [Curve (a) from Newing,
M.J., Trans. Faraday Soc., 46, 613, 1950; (b) and (d) from Bawn, C.E.H., Freeman, R.F.J., and Kamaliddin, A.R.,
Trans. Faraday Soc., 46, 677, 1950; (c) from Gee, G. and Orr, W.J.C., Trans. Faraday Soc., 42, 507, 1946; and
Gee, G., J. Chem. Soc., p. 280, 1947.]
occur between segments within such an excluded volume. The expression for the excess, relative, partial
molar-free energy for these interaction is given by the relation:
∆G1 = RT (κ1 − ψ1 ) ν22
(12.35)
κ1 and χ1 are heat and entropy parameters also given by
∆H1 = RT κ1 ν22
(12.36)
∆S1 = R ψ1 ν22
(12.37)
If x is assumed to be infinite, Equation 12.29 reduces to
[
∆G1 = RT ln (1 − ν2 ) + ν2 + χ1 ν22
]
(12.38)
Expanding the logarithmic term and, as before, neglecting terms of order higher than 2 (i.e., ln (1 – ν2) =
–ν2 –
ν22
2
– …), Equation 12.38 becomes
1
∆G1 = − RT − χ1 ν22
2
By comparing Equations 12.35 and 12.39, it is obvious that
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(12.39)
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SOLUTION PROPERTIES OF POLYMERS
κ1 − ψ 1 =
1
− χ1
2
(12.40)
Table 12.5 Behavior of Polymer Molecules with Change in Thermodynamic Parameters
Thermodynamic
Parameter
κ1 = 0 (θ = 0)
κ1 > 0 (T < θ)
κ1 = ψ1 (T = θ)
κ1 < 0 (T > θ)
Behavior of Polymer Molecules
A thermal solvent, δ∆G>0; increase in segment concentration within volume element causes a
decrease in entropy; solution wants to dilute itself; polymer coils move apart
Polymer–polymer contacts preferred to polymer–solvent contacts; spontaneous concentration
δ∆G = 0; excluded volume effects eliminated; polymer in an unperturbed state
Exothermic solution; δ∆G = 0; spontaneous dilution; expansion of polymer coil occurs as a result
of interaction with solvent
The deviations from ideality in a polymer solution can be eliminated by selecting a temperature where
∆H1 = T∆S1 or when κ1 = ψ1. The temperature at which these conditions prevail is called the Flory or
theta temperature, θ, and is defined by
θ=
κ1 T
ψ1
(12.41)
It follows that
ψ1 − κ1 = ψ1 (1 − θ T)
(12.42)
∆G1 = − RT ψ1 (1 − θ T) ν22
(12.43)
and
We note for emphasis (also from Equation 12.43) that at the temperature T = θ the excess, relative,
partial molar-free energy, ∆G1, due to polymer–solvent interactions is zero and deviations from ideality
vanish.
The change in free energy δ (∆G) in a volume element when two molecules are brought together
depends algebraically on the magnitude of ψ1 – κ1 or ψ1 (1 – θ/T). Since the entropy of dilution is
usually positive, the sign of the free energy change will depend on the relative magnitudes of ψ1 and κ1
or if θ/T is greater than unity. Table 12.5 summarizes the expected behavior of polymer molecules with
thermodynamic parameters.
It follows from the above discussion that as the solvent is made poorer, i.e., as ψ1 (1 – θ/T) decreases,
the excluded volume shrinks and at T = θ it disappears entirely. In other words, as the solvent becomes
poorer, polymer–polymer repulsion diminishes and at the θ-point the net interaction becomes zero.
Where T < θ, polymer molecules attract each other and the excluded volume is negative. When the
temperature is much lower than the θ-point, precipitation occurs.
D. OSMOTIC PRESSURE OF POLYMER SOLUTIONS
Osmotic pressure, as indicated earlier, is one of the quantities that can be obtained experimentally from
the Flory–Huggins and Flory–Krigbaum theories. Before we illustrate how thermodynamic parameters
characteristic of polymers can be derived from osmotic pressure measurements, let us first explain very
briefly the basis of these measurements.13
Consider the apparatus shown schematically in Figure 12.11. The semipermeable membrane, represented by the dashed line, allows the passage of solvent but not the solute. Suppose in the first instance
that both sides of the tube contain only the pure solvent. At equilibrium the levels of the liquid in both
arms would be at the same height and the external pressure would be PA. In this case, the chemical
potential of the solvent on both sides would be the same. Suppose a solute is now added to the right-
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POLYMER SCIENCE AND TECHNOLOGY
Figure 12.11 Schematic representation of an osmometer.
hand side. Since it cannot pass through the semipermeable membrane, it must remain on right-hand
side. The chemical potential of the solvent on the right-hand side (solution) is now less than that of the
solvent on the left-hand side (pure solvent). If the external pressure on the right-hand side is maintained
at PA, the liquid level on the right-hand side will rise as the solvent passes from the left (higher chemical
potential) to the right (lower chemical potential) to equalize the chemical potential on both sides.
However, this flow of solvent can be prevented if the external pressure on the solution is increased so
as to keep the liquid levels the same on both sides. The additional pressure is the osmotic pressure, π,
of the solution. It arises as the driving force for solvent flow in response to the reduction of the chemical
potential of the solvent due to the addition of a solute.
From Equation 12.32, the osmotic pressure is given by
π=
RT ν2 1
1
+
− χ1 ν22 + ν23 + L
V1 x 2
3
(12.32)
It is generally more convenient to replace volume fraction with the concentration, C, of the solution
expressed in weight per unit volume:
ν2 = C ν2
(12.44)
where ν2 is the (partial) specific volume of the polymer. Since x = V2/V1, then
ν2
cν
C
= 2 =
xV1 xV1 M 2
(12.45)
where M2 is the molecular weight or the polymer. For heterogeneous polymers, M2 is replaced by the
number-average molecular weight Mn. From Equation 12.45
ν2 =
Copyright 2000 by CRC Press LLC
c x V1
M2
(12.46)
325
SOLUTION PROPERTIES OF POLYMERS
x=
ν2 M 2
V1
(12.47)
Substituting Equations 12.46 and 12.47 successively in Equation 12.32 and rearranging terms yields
2
3
π RT 1
ν2 M 2 c + ν2 M 2 c2 + L
=
1 + − χ1
c M2
V1
2
3 V1
(12.48)
In applications to osmotic data, Equation 12.48 is most frequently preferred in the following forms:
π
= RT A1 + A 2 c + A 3 c 2 + L
c
[
π RT
=
1 + Γc + gΓ 2 c2 + L
c M2
[
]
]
(12.49)
(12.50)
where A1 = 1/M2 and A2 and Γ are the second virial coefficients given by
Γ=
ν22 1
− χ1
V1 2
gΓ 2 =
ν23 M 2
3 V1
(12.51)
(12.52)
Osmotic pressure data can, therefore, be used for the determination of the number-average molecular
weight, Mn, or the solvent–polymer interaction parameter χ1. This depends, of course, on knowing the
values of the densities and specific volumes of the polymer and the solvent. In good solvents, g is
approximately 1/4 in which case Equation 12.50 becomes
π
c
12
RT
=
M2
12
1 + 1 Γc
2
(12.53)
In poor solvents, g ≅ 0. In either case, an appropriate plot of π/c vs. c gives M2(Mn) and χ1.
Other thermodynamic parameters can be obtained from osmotic pressure. For example, the chemical
potential of the solvent in the solution is given by –χ/πV1. From the foregoing discussion, it is evident
that the thermodynamic behavior of the dilute polymer solution depends on the following factors:
1. Molecular weight
2. The interaction parameter ψ1 and κ1 or ψ1 and θ, which characterize the segment–solvent interactions
3. The size or configuration of the molecules in solution
The first factor is usually determined from the coefficient A1. A2 (or Γ2) depends on all three factors.
Therefore, to evaluate the thermodynamic functions that depend on A2, it is necessary to determine the
size of the molecule in solution independently. The parameter α is related to the thermodynamic quantities
according to Equation 12.54:
α 5 − α 3 = 4 C M ψ1 (1 − θ T) M1 2
(12.54)
where CM represents all the numerical and molecular constants. The expansion factor α3 and r02/M may
be determined from suitable measurements of intrinsic viscosities. The quantity ψ1(1 – θ/T) may then
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POLYMER SCIENCE AND TECHNOLOGY
be deduced from the relevant equation. If measurements are made over a limited range of temperature
in the vicinity of θ, ψ1(1 – θ/T) may be resolved into its components θ and ψ1 from a plot of A2 against
the reciprocal of the absolute temperature.
Figure 12.12 Concentration dependence of osmotic pressure of solutions of polystyrene in toluene at 30°C.
Numbers on each curve denote polymer molecular weights. (From Flory, P.J., Principles of Polymer Chemistry,
Cornell University Press, Ithaca, NY, 1953. With permission.)
Typical results of osmotic pressure measurements are shown in Figure 12.12. The experimental data
are in consonance with the predicted positive curvature in good solvents. However, these data also show
that the quantity ψ1(1 – θ/T) decreases with increasing molecular weight. This is contrary to theory,
which predicts that these thermodynamic parameters should characterize the inherent segment–solvent
interaction independent of the entire molecular structure. The theory predicts that α increases without
limit with molecular weight and that χ1 is constant over a wide range of polymer concentrations. However,
experimental data show that χ1 increases with increasing volume fraction ν2. In spite of these shortcomings, the Flory–Huggins intermolecular interaction theory is in reasonably satisfactory agreement with
experimental data within the approximations made in the theory. It provides a semiquantitative description
of polymer solutions from which parameters characteristic of polymers may be derived. Table 12.6 gives
values of χ1 for some polymer–solvent systems.
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SOLUTION PROPERTIES OF POLYMERS
Table 12.6 Polymer–solvent Interaction
Parameter at 25°C
Polymer
Poly(dimethylsiloxane)
Polyisoprene (natural rubber)
Poly(methyl methacrylate)
Polystyrene
Solvent
χ1
Cyclohexane
Chlorobenzene
Benzene
Chloroform
Tetrahydrofuran
Ethylbenzene
Methyl ethyl ketone
Cyclohexane
0.42
0.47
0.42
0.377
0.447
0.40
0.47
0.505
From Wolf, B.A., in Polymer Handbook, 2nd ed., Brandrup, J.
and Immergut, E.M., Eds., John Wiley & Sons, New York, 1967.
With permission.
Example 12.3: The data on the osmotic pressure of a sample of poly(vinyl acetate) in methyl ethyl
ketone at 25°C are shown below:
Weight Fraction of Polymer
Pressure in cm of Solution
0.0021
0.0032
0.0057
0.0061
0.0082
0.0083
0.0093
0.0100
0.0114
0.0122
0.40
0.61
1.23
1.44
2.10
2.25
2.52
2.76
3.54
3.73
The densities of the polymer and solvent are 1.190 and 0.800, respectively; the solution densities can
be calculated by assuming additivity of the volumes of the components. Using both linear and squareroot plots, calculate the polymer molecular weight and the second virial coefficients, A2 and Γ.
Solution: Basis: 1 g of solution:
Vol of solution Vs =
ω2 1 − ω2
+
in cm 3
ρ2
ρ1
(
)
where ω2 = weight fraction of polymer; ρ1 and ρ2 are the densities of the solvent and polymer, respectively.
Concentration of Solution, C =
ω2
g cm 3
Vs
(
)
From these relations, data can be transformed into a Figure E12.3A,B. Now
1
π
= RT
+ A 2 c + A3 c2 + L
c
M
n
π
c
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12
RT
=
Mn
12
[1 + 0.5 Γ c]
328
POLYMER SCIENCE AND TECHNOLOGY
400
375
(a)
350
Π (g/cm2)
C (g/cm3)
325
300
275
250
225
200
0.002
0.004
0.006
0.008
0.010
0.008
0.010
C (g/cm3)
20
(b)
1/
2
(g/cm2)
(g/cm3)
18
16
Π
C
14
12
10
0.002
0.004
0.006
C
(g/cm3)
Figure E12.3
From Figure E12.3a,
RT
= intercept on π/C as c → 0
Mn
Intercept on π c = 194
Mn =
cm solution
g cm 3
1
RT
cm 3 atm
= 82.06
(298 K)
cm solution
intercept
mol − K
194
g cm 3
cm 3 − 1033 cm H 2O
1
= 82.06
(298 K)
cm H 2O
mol − K
194 × 0.8
g cm 3
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329
SOLUTION PROPERTIES OF POLYMERS
Note:
l atm = 1033 cm H2O; cm solution xρs = cm H2O. Since the solution is dilute it is assumed that
the density of solution is approximately that of the solvent.
M n = 1.63 × 10 5 g mol
Slope = RTA 2
(
)
2
19648 cm solution g cm 3
slope
A2 =
=
RT
82.06 cm 3 − atm mol − K (298 K)
(
=
(
)
(
)
− cm H O mol − K × 1033) (298 K)
19648 × 0.8 cm H 2O g cm 3
82.06 cm 3
2
2
= 6.2 × 10 −4 mol − cm 3 g2
From Figure E12.3b,
RT
Slope = 563 = 0.5 Γ
Mn
Γ=
RT
M
n
12
563
0.5
RT
M
n
12
−1 2
π
= intercept at
c
12
as c → 0
= 14.22
Γ=
563
= 79.2 cm 3 g
0.5 × 14.22
V. SOLUTION VISCOSITY
Rheology by definition is the science of deformation and flow of matter. Rheological measurements
provide useful behavioral and predictive information for various products in addition to knowledge of
the effects of processing, formulation changes, and aging phenomena. Material processability can also
be determined through rheological studies. Rheology deals with those properties of materials that
determine their response to mechanical force. For solids, as we shall see in subsequent chapters, this
involves elasticity and plasticity. For fluids, on the other hand, rheological studies involve viscosity
measurements. Viscosity is a measure of the internal friction of a fluid. For example, it has been observed
that even at low concentrations of a dissolved polymer, the viscosity of a solution relative to that of the
pure solvent is increased appreciably. This is due to the unusual size and shape of polymer molecules
and the nature of their solutions. Thus measurements of the viscosity of polymer solutions can provide
information about monomer molecular weight, molecular weight distribution, and other material characterization parameters. Before we deal with the relation between solution viscosity and polymer
characterization parameters, we discuss briefly the various terms used to describe viscosity.
A. NEWTON’S LAW OF VISCOSITY16,17
Consider a fluid, which may be a gas or liquid, contained between two large parallel plates of area A
and separated by the distance Y (Figure 12.13). The system is initially at rest. Now suppose the lower
plate is set in motion in the x-direction at a constant velocity. The fluid gains momentum with time, and
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POLYMER SCIENCE AND TECHNOLOGY
Figure 12.13 Laminar velocity profile for fluid between two plates.
at steady state a constant force, F, is required to maintain the motion of the lower plate. For laminar
flow, this force is given by Equation 12.55:
F
V
=η
A
Y
(12.55)
According to this equation, the force per unit area is proportional to the velocity decrease in the distance
Y. This constant of proportionality, η, is called the viscosity of the fluid. It is desirable and convenient
to rewrite Equation 12.55 in a form that permits us to give a molecular interpretation to the meaning of
viscosity (Equation 12.56):
τ yx = − η
dVx
dy
(12.56)
Equation 12.56 states that the shear stress is proportional to the negative of the local velocity gradient.
This is Newton’s law of viscosity, and fluids that exhibit this behavior are referred to as Newtonian
fluids. According to this law, in the neighborhood of the surface of the moving plate (i.e., at y = 0), the
fluid acquires a certain amount of x-momentum. This fluid, in turn, transmits some of its momentum to
the adjacent layer of fluid causing it to remain in motion in the x-direction; in effect, the x-momentum
is transmitted in the y-direction. The velocity gradient is a measure of the speed at which intermediate
layers move with respect to each other. For a given stress, fluid viscosity determines the magnitude of
the local velocity gradient. Fluid viscosity is due to molecular interaction; it is a measure of a fluid’s
tendency to resist flow, and hence it is usually referred to as the internal friction of a fluid.
Using the chain rule, the velocity gradient in Equation 12.56 can be interpreted differently:
dV
d dx d dx dγ «
=γ
=
=
=
dy dy dt dt dy dt
(12.57)
where γ is the strain rate. In a more general form, Equation 10.56 becomes
τ = − ηγ«
(12.58)
In this form, Newton’s law simply states that for laminar flow, the shear stress needed to maintain
the motion of a plane of fluid at a constant velocity is proportional to the strain rate. At a given
temperature, the viscosity of a Newtonian fluid is independent of the strain rate (Figure 12.14). Fluids
that do not obey Newton’s law of viscosity are known as non-Newtonian fluids. For non-Newtonian
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SOLUTION PROPERTIES OF POLYMERS
331
fluids, when the strain rate is varied, the shear stress does not vary in the same proportion, i.e., the
Figure 12.14 Behavior of Newtonian fluids.
Figure 12.15 Some types of non-Newtonian behavior.
viscosity is not independent of the strain rate. In physical terms, non-Newtonian behavior means that as
molecules pass each other, their size, shape, and cohesiveness determine how much force is required to
maintain the movement. When the strain rate is changed, molecular alignment may change also as will
the force required to maintain motion. There are several types of deviation from Newtonian behavior.
Each is characterized by the way the fluid viscosity changes in response to variations in the strain rate
(Figure 12.15). Pseudoplastic fluids display a decrease in viscosity with increasing strain rate, while a
dilatant fluid is characterized by an increase in viscosity with increasing strain rate. For fluids that exhibit
plastic behavior, a certain amount of stress is required to induce flow. The minimum stress necessary to
induce flow is frequently referred to as the yield value. In addition, some fluids will show a change of
viscosity with time at a constant strain rate and in the absence of a chemical reaction. Two categories
of this behavior are encountered: thixotropy and rheopexy. A thixotropic fluid undergoes a decrease in
viscosity, whereas a rheopectic fluid displays an increase in viscosity with time under constant strain
rate (Figure 12.16). Table 12.7 gives examples of fluids that display Newtonian and non-Newtonian
behavior.
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POLYMER SCIENCE AND TECHNOLOGY
Figure 12.16 Change of viscosity with time under constant strain rate.
Table 12.7 Examples of Newtonian and Non-Newtonian Fluids
Viscosity Type
Newtonian
Non-Newtonian
Pseudoplastic
Dilatant
Plastic
Thixotropic
Rheopectic
Example
All gases, water, thin motor oils
Paints, emulsions, dispersions
In fluids containing high levels of deflocculated solids such as clay slurries, candy compounds,
corn starch in water, sand/water mixtures
Tomato catsup
Greases, heavy printing inks, paints
Rarely encountered
B. PARAMETERS FOR CHARACTERIZING POLYMER SOLUTION VISCOSITY12
The viscous flow of a polymer solution involves a shearing action in which different layers of the solution
move with differing velocities. As we observed earlier, there is a pronounced increase in the viscosity
of a polymer solution relative to that of the pure solvent even at low concentrations of the polymer. In
this respect, the polymer solute behaves as a colloidal dispersion, which is known to retard the flow of
adjacent layers of a liquid under shearing force. For spherical colloidal particles, the viscosity of the
solution, η, relative to that of the pure solvent, η0, is referred to as the relative viscosity, given by
ηr = η η0
(12.59)
The flow of fluids through a tube of uniform cross-section under an applied pressure is given by the
Hagen–Poiseuille’s law:
Q=
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π R4 ∆ P
8 ηL
(12.60)
333
SOLUTION PROPERTIES OF POLYMERS
where Q is the volume flow rate, ∆P is the pressure drop across the tube of length L and radius R. Some
of the assumptions made in the derivation of this law include
1.
2.
3.
4.
The flow is laminar, which means that the dimensionless quantity called Reynold’s number, Re, < 2100.
The fluid is incompressible, i.e., its density is constant.
The flow is independent of time, i.e., steady-state conditions prevail.
The fluid is Newtonian.
The viscosity of a liquid or solution can be measured by using a viscometer whose design is based
on the Hagen–Poiseuille law. Essentially, this involves the measurement of the flow rate of the liquid
through a capillary tube which is part of the viscometer. Consequently, by measuring the flow time of
the solution, t, and that of the pure solvent, t0, the relative viscosity can be determined:
ηr = η η0 = t t 0
(12.61)
As indicated above, the viscosity of the polymer solution is always greater than that of the pure solvent.
This fractional increase in the viscosity resulting from the dissolved polymer in the solvent is referred
to as the specific viscosity ηsp, given by
Table 12.8 Various Viscosity Terms
Viscosity Term
Expression
Relative viscosity
Specific viscosity
Reduced viscosity
ηr = η/η0 = t/t0
ηsp = (η – η0)/η0 = ηr – 1
ηsp/C = (ηr – 1)/C
Intrinsic viscosity
[η] = lim ηsp/C
ηsp =
c→0
η − η0
= ηr − 1
η0
(12.62)
According to Einstein’s viscosity relation for rigid spherical particles in solution,
η − η0
n
= 2.5 2 Ve
V
η0
(12.63)
4π
- R3e; Re is the radius of an
where n2 /V is the number of polymer molecules per unit volume and Ve = ----3
equivalent hydrodynamic sphere that would enhance the viscosity of the solvent medium to the same
extent as would the actual polymer molecule. The quantity n2/V may be written as
n2 C NA
=
V
M
(12.64)
where C and M are the concentration and molecular weight, respectively, of the polymer, and NA is
Avogadro’s number. It is obvious from Equations 12.63 and 12.64 that both the relative viscosity and
specific viscosity increase with increasing concentration of the polymer. The specific viscosity normalized
with respect to the concentration, ηsp /C, is referred to as the reduced specific viscosity or, simply, reduced
viscosity. It measures that capacity with which a given polymer enhances the specific viscosity. The
intrinsic viscosity [η] is the limiting value of the reduced viscosity at infinite dilution:
ηsp
[η] = lim
c→ o
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c
(12.65)
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POLYMER SCIENCE AND TECHNOLOGY
When ηr < 2, it has been found that a linear relation exists between the reduced viscosity and polymer
concentration. For a given polymer–solvent system, this linear dependence is described adequately by
the equation
ηsp
c
= [ η] + k1 [ η] c
2
(12.66)
where k1 is referred to as the Huggins constant, with a value usually in the range 0.35 to 0.40. Table 12.8
summarizes the various viscosity terms described above.
C. MOLECULAR SIZE AND INTRINSIC VISCOSITY
The Einstein viscosity relation given by Equation 12.62 may be written as
η − η0
N C
= 2.5 A
V
M e
η0
(12.67)
or
[η] =
η − η0
N
N 4π 3
= 2.5 A Ve = 2.5 A
R
η0 C
M
M 3 e
(12.68)
Equation 12.67 predicts that the specific viscosity is proportional to the volume of the equivalent
hydrodynamic sphere. The Einstein viscosity relation was derived for rigid spherical particles in solution.
However, real polymer molecules are neither rigid nor spherical. Instead the spatial form of the polymer
molecule in solution is regarded as a random coil. Theories based on this characteristic form of polymer
molecules have resulted in the expression
[η] = Φ (r 2 )
32
M
(12.69)
where φ is considered to be a universal constant with a value of 2.1 (±0.2) × 1023, r 2 is the mean-square
end-to-end distance of the polymer coil expressed in cm, and [η] is in cm3/g. According to Equation 12.68,
the intrinsic viscosity is proportional to the ratio of the effective hydrodynamic volume of the molecule
to its molecular weight. Specifically, it states that the effective volume is proportional to the linear
dimensions of the randomly coiled polymer chain. Therefore, to understand the factors that influence
the intrinsic viscosity, the quantity in Equation 12.68 may be separated into its component parts. Recall
from our discussion in Section III.B that the net effect of long-range interaction is to expand the actual
chain dimension (r02)1/2 by an expansion factor α given by the relation:
(r )
2
12
( )
(12.13)
)
(12.70)
= α r02
12
Equation 12.69 may therefore be rewritten as
[η] = Φ (r02
M
32
M1 2 α 3
For a linear polymer of a given structural unit, the quantity r20/M is independent of M. Consequently,
Equation 12.70 becomes
[η] = K M1 2 α 3
where
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(12.71)
335
SOLUTION PROPERTIES OF POLYMERS
(
K = Φ r02 M
)
32
(12.72)
K is a constant independent of the polymer molecular weight and of the solvent.
From Equation 12.71, the intrinsic viscosity depends on the molecular weight as a result of the factor
M1/2 and also through the dependence of the expansion factor α3 on molecular weight. By choosing a
theta-solvent or θ temperature, the influence of the molecular expansion due to intramolecular interactions
can be eliminated. Under these conditions, α = 1, and the intrinsic viscosity depends only on the molecular
weight. Thus Equation 12.71 is reduced to:
[η] θ = K M1 2
(12.73)
This relation has been confirmed experimentally. It follows, therefore, that since Φ is regarded as a
universal constant, the average dimensions of polymer molecules in solution can be estimated from
knowledge of their intrinsic viscosities and molecular weight (Equation 12.71). Specifically, the unperturbed dimensions can be calculated from the value of K.
Figure 12.17 Intrinsic viscosity–molecular weight relationship. (From Allocock, H.R. and Lampe, F.W., Contemporary Polymer Chemistry, Prentice-Hall, Englewood Cliffs, NJ, 1981. With permission.)
D. MOLECULAR WEIGHT FROM INTRINSIC VISCOSITY
Polymers possess the unique capacity to increase the viscosity of the solvent in which they are dissolved.
Within a homologous series of linear polymers, the higher the molecular weight the greater the increase
in viscosity for a given polymer concentration. In other words, this capacity to enhance viscosity or
intrinsic viscosity is a reflection of the molecular weight of the dissolved polymer. Consequently, intrinsic
viscosity measurements provide a tool for characterization of polymer molecular weight. However, since
intrinsic viscosity does not provide absolute values of molecular weight, the relation between intrinsic
viscosity and molecular weight has to be established empirically by comparison with molecular weights
determined from absolute methods such as osmometry, light scattering, and ultracentrifugation. A linear
relation has been found to exist between the logarithms of the intrinsic viscosities of different molecular
weight fractions of a given polymer and the logarithms of the molecular weights of these fractions. This
is illustrated for polyisobutylene in cyclohexane at 30°C and for polystyrene in cyclohexane at 35°C
and in ethylethylketone at 25°C, as shown in Figure 12.17. The slopes of these curves for a given polymer
depend on the solvent and, for a given polymer–solvent pair, on the temperature. It has also been
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POLYMER SCIENCE AND TECHNOLOGY
established that the slopes of such plots for all polymer–solvent systems fall within the range of 0.5 to
1.0. The linear relation between log[η] and log M may then be written as
[η] = K Ma
(12.74)
where K and a are constants determined from the intercept and slope of plots such as in Figure 12.17.
The relation given in Equation 12.74 is referred to as the Mark–Houwink equation. Using the equation,
it is possible to calculate the molecular weight from intrinsic viscosity measurements as long as K and
a have been established for a particular temperature.
It must be reemphasized that the Mark–Houwink equation applies to fractionated samples of a given
polymer. This means that, strictly speaking, it covers only a narrow molecular weight range. However,
it is relatively easier in practice to use intrinsic viscosity measurements for the determination of the
molecular weights even for unfractionated polymers. For such molecularly heterogeneous polymers, the
appropriate relation becomes
[η] = K M νa
(12.75)
where Mν is the viscosity average molecular weight given by
Mν =
[∑ ω M ]
i
a
i
1a
=
∑N M
∑N M
1+ a
i
i
i
i
1a
(12.76)
Table 12.9 Intrinsic Viscosity–Molecular Weight Relationship, [η] = KMa
Polymer
Polybutadiene
Natural rubber
Polyethylene
Low pressure
High pressure
Polyisobutylene
Polypropylene (atactic)
Polypropylene (isotactic)
Poly(methyl methacrylate)
Polystyrene (atactic)
Polystyrene (isotactic)
Solvent
Temperature (°C)
Cyclohexane
Benzene
Toluene
Benzene
Toluene
40
30
30
30
25
Decalin
Decalin
Benzene
Cyclohexane
Diisobutylene
Toluene
Decalin
Benzene
Cyclohexane
Decalin
Acetone
Benzene
Chloroform
Benzene
Cyclohexane
Benzene
Toluene
Chloroform
135
70
25
25
25
25
135
25
25
135
25
25
25
20
34.5
30
30
30
Mol-Wt Range × 10–4
4–17
5–50
5–16
8–28
7–100
3–100
0.2–3.5
0.05–126
14–34
0.4–2.5
14–34
2–39
6–31
6–31
2–62
2–780
2–740
40–330
0.6–520
14–200
4–37
15–71
9–32
K × 103 (ml/g)
a
28.2
33.7
29.4
18.5
50.2
0.70
0.715
0.753
0.74
0.667
67.7
38.73
83.0
40.0
130.0
87.0
15.8
27.0
16.0
11.0
5.3
5.5
3.4
12.3
84.6
10.6
9.3
25.9
0.67
0.738
0.53
0.72
0.50
0.56
0.77
0.71
0.80
0.80
0.73
0.76
0.83
0.72
0.50
0.735
0.72
0.734
From Kurata, M., Tsunashima, Y., Iwama, M., and Kamada, K., in Polymer Handbook, 2nd ed., Brandrup, J. and Immergut,
E.H., Eds., John Wiley & Sons, New York, 1975. With permission.
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337
SOLUTION PROPERTIES OF POLYMERS
where ωi = ci /c = weight fraction of species i; Ni, Mi, and Ci are the number of molecules, molecular
weight, and concentration, respectively, of the same species; and C = ∑ci is the total concentration of
all species. Table 12.9 lists the values for K and a for some selected polymer–solvent systems.
Example 12.4: Explain the variation in the value of K with temperature for polystyrene measured in
different solvents.
Solvent
θ(K)
K × 104 (at T = θ)
Cyclohexane
Methycyclohexane
Ethylcyclohexane
307
343.5
343
8.2
7.6
7.5
Solution: K is a constant that is essentially independent of the molecular weight of the polymer and
the character of the solvent medium, as amply demonstrated by the above data. However, K shows a
decrease with increasing temperature. From Equation 12.72, K is proportional to the factor r20/M. We
recall that the unperturbed root-mean-square end-to-end distance (r20)1/2 is expanded invariably to greater
dimensions relative to completely free rotation as a result of the effects of hindrances to free rotation.
As the temperature is increased, the tendency to completely free rotation is enhanced as the effects of
these hindrances are diminished. Consequently, K also decreases.
Example 12.5: Given the following values of ηr for a polyisobutylene sample of molecular weight
1,500,000:
Solvent
Cyclohexane
Diisobutylene
Benzene
Temp (°C)
C = 0.05
0.10
0.15
0.20g/dl
30
20
25
1.282
1.173
1.066
1.611
1.365
1.136
1.988
1.578
1.209
2.412
1.809
1.287
a. Determine [η] and k′ in each solvent.
b. Calculate (r20 /M)1/2, C∞ and the value αn in each solvent. Note that 25°C is the θ-temperature
in benzene and assume Φ = 2.6 × 1021 in this case.
Solution:
ηsp = ηr − 1
From the plot of ηsp/C vs. C, i.e., Figure E12.5
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POLYMER SCIENCE AND TECHNOLOGY
Figure E12.5
Hexane: [ η] = 5 ⋅ 15 slope = k ′ [ η] = 9.75
2
k ′ = 9.75 (5.15) = 0.368
2
Hexane
Diisobutylene
Benzene
η]
[η
Slope
k′′
5.15
3.20
1.28
9.75
4.25
0.8
0.368
0.415
0.488
b. Since 25°C θ is the temperature in benzene,
[η] θ = K M1 2 = 1.28
K=
[η] θ
M
12
1.28
=
r2
K = Φ 0
M
(1.5 × 10 )
6 12
= 1.045 × 10 −3
32
or
r2
0
M
32
=
K 1.045 × 10 −3
=
2.6 × 10 21
Φ
= 0.402 × 10 −24
r2
0
M
Copyright 2000 by CRC Press LLC
12
= 0.738 × 10 −8
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SOLUTION PROPERTIES OF POLYMERS
C∞ =
r02
nl 2
(
)
(
) (1.5 × 10 )
r02 = 0.738 × 10 −8
= 0.738 × 10 −8
2
M
2
6
= 0.817 × 10 −10
CH3
Polyisobutylene:
CH2
C
CH3
n
(Str. 3)
Molecular weight of monomer = 56 = Mo.
n = 2 DP =
2 M 2 × 1.5 × 10 6
=
Mo
56
= 5.357 × 10 4
l = 1.54 × 10 −8 cm
C∞ =
0.817 × 10 −10
5.357 × 10 6 × 2.372 × 10 −16
= 6.43
[η] = [η] θ α n3
Cyclohexane:
(
[ ]
α n = [ η] η0
)
13
θ
5.15
=
1.28
= 1.59
13
Dilsobutylene:
3.20
αn =
1.28
Benzene:
1.28
αn =
1.28
VI.
13
= 1.36
13
= 1.0
PROBLEMS
12.1. Which of the following is the most suitable solvent for (a) natural rubber and (b) polyacrylonitrile:
n-pentane, toluene, o-dichlorobenzene, nitroethane, and nitromethane? The densities of natural rubber
and polyacrylonitrile are 1.1 and 1.15, respectively.
12.2. Explain the following observations.
a. An excellent example of a homogeneous polymerization is the bulk polymerization of methyl methacrylate, while heterogeneous polymerization is exemplified by bulk polymerization of vinyl chloride.
Copyright 2000 by CRC Press LLC
340
POLYMER SCIENCE AND TECHNOLOGY
b. Polyacrylonitrile is prepared industrially either by precipitation polymerization of acrylonitrile in
water or its solution polymerization in dimethyl formamide.
12.3. Show that the end-to-end distance of the freely jointed chain is expanded by a factor of 2 when there
is restriction to bond angles. Assume that the bond angle is tetrahedral.
12.4. The root-mean-square end-to-end distance of poly(acrylic acid) with molecular weight 1,000,000 in a
θ solvent at 30°C is 670 Å. Taking the C–C bond length as 1.53 Å and all backbone bond angles as
tetrahedral, calculate:
a.
b.
c.
d.
Flory’s characteristic ratio
The Stockmayer–Kurata ratio
The Kratky–Porod persistence length
The contour length of the planar zigzag
12.5. How does the root-mean-square end-to-end distance of a flexible polymer depend on:
a. Molecular weight at the Flory temperature θ
b. Temperature at the Flory point
c. Molecular weight, well above the Flory temperature for a good solvent
12.6. Calculate the vapor pressure over a 50% solution (by volume) of poly(vinyl acetate) in methylethyl
ketone at 25°C using the lattice theory and assuming that the value of χ1 derived from the dilute solution
data is valid in the concentrated solution. The vapor pressure of pure methylethyl ketone at 25°C is
100 mm Hg.
12.7. The size of a polyisobutylene molecule in the pure material is given experimentally as
r2
0
M
12
= 0.795 at 24°C
= 0.757 at 95°C
where r0 is in angstroms and M is molecular weight. Compare these results to those obtained by assuming
free rotation about the valence bonds. Explain why the experimental values:
a. Are higher than the calculated values
b. Decrease with increasing temperature
If the root-mean-square end-to-end distance of the polymer molecule is 1000 Å, what is its molecular
weight at 24°C?
12.8. Compute the root-mean-square end-to-end length for a polystyrene molecule having a molecular weight
of 106. Assume free rotation on the valence cone.
12.9. Polyisobutylene has a molecular weight of 885,000 and density 0.8 g/cm3. If
r2
0
M
12
= 800 × 10 −11
a. What is the approximate contour length of this polymer molecule (length of the planar zigzag
structure)?
b. What is the approximate size of the molecule if a compact shape is assumed? Compact shape density
is 0.8 g/cm3 (what is the radius of an equivalent sphere).
c. What is the approximate size of the molecule if a hindered random coil is assumed?
d. Discuss the relative magnitudes of parts b and c.
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SOLUTION PROPERTIES OF POLYMERS
12.10. The ratios of the unperturbed end-to-end dimensions measured at 60°C to that calculated assuming
completely free rotation (r20 /-r 20f ) 1/2 for natural rubber and gutta-percha are 1.71 and 1.46, respectively.
Explain.
12.11. The following data are for a narrow molecular weight fraction of poly(methyl methacrylate) in acetone
at 30°C (density acetone = 0.780 g/cm3). Plot appropriately and estimate [η], k′ (Huggins constant),
Mn, and the second virial coefficient. Knowing that [η]θ = 4.8 × 10–4 M0. 5 for this polymer in a theta
solvent at 30°C, calculate from these data (r20)1/2, (r22)1/2 and α, the expansion factor.
Concentration C,
(g/100 ml)
Osmotic Pressure
(cm solvent)
ηr, Relative Viscosity
0.275
0.338
0.344
0.486
0.96
1.006
1.199
1.536
1.604
2.108
2.878
0.457
0.592
0.609
0.867
1.756
2.098
2.710
3.728
3.978
5.919
9.713
1.170
—
1.215
—
1.629
—
1.892
—
2.330
2.995
—
12.12. The following data are available for polymers A and B in the same solvent at 27°C.
Conc, CA
(g/dl)
Osmotic
pressure,
(cm of solvent)
Conc, CB
(g/dl)
Osmotic
Pressure
(cm of solvent)
0.320
0.660
1.000
1.900
0.70
1.82
3.10
9.30
0.400
0.900
1.400
1.800
1.60
4.44
8.95
13.0
Solvent density = 0.85 g/cm3; polymer density = 1.15 g/cm3.
a. Estimate Mn and the second virial coefficient for each polymer.
b. Estimate Mn for a 25:75 mixture of A and B.
c. If Mw/-Mn = 2.00 for A and for B, what is Mw/-Mn for the mixture in part b?
REFERENCES
1. Hildebrand, J.H. and Scott, R.L., The Solubility of Nonelectrolytes, Van Nostrand-Reinhold, New York, 1950.
2. Burrel, H. and Immergut, B., in Polymer Handbook, Brandrup, J. and Immergut, E.M., Eds., John Wiley & Sons,
New York, 1967.
3. Hoy, K.L., J. Paint Technol., 42, 76, 1970.
4. Kurata, M., Tsunashima, Y., Iwama, M., and Kamada, K., Viscosity molecular-weight relationships and unperturbed
dimensions of linear chain molecules, in Polymer Handbook, 2nd ed., Brandrup, J. and Immergut, E.H., Eds., John
Wiley & Sons, New York, 1975.
5. Flory, P.J., J. Chem. Phys., 10, 51, 1942.
6. Huggins, M.L., Ann. N.Y. Acad. Sci., 43, 1, 1942.
7. Huggins, M.L., J. Phys. Chem., 45, 151, 1942.
8. Huggins, M.L., J. Am. Chem. Soc., 64, 1712, 1942.
9. Newing, M.J., Trans. Faraday Soc., 46, 613, 1950.
10. Bawn, C.E.H., Freeman, R.F.J., and Kamaliddin, A.R., Trans. Faraday Soc., 46, 677, 1950.
11. Gee, G. and Orr, W.J.C., Trans. Faraday Soc., 42, 507, 1946.
12. Gee, G., J. Chem. Soc., p. 280, 1947.
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13. Klotz, M., Chemical Thermodynamics, Prentice-Hall, Englewood Cliffs, NJ, 1950.
14. Flory, P.J., Principles of Polymer Chemistry, Cornell University Press, Ithaca, NY, 1953.
15. Wolf, B.A. in Polymer Handbook, 2nd ed., Brandrup, J. and Immergut, E.M., Eds., John Wiley & Sons, New York,
1967.
16. Bird, R.B., Stewart, W.E., and Lightfoot, E.N., Transport Phenomena, John Wiley & Sons, New York, 1960.
17. Anon., More Solutions to Sticky Problems, Brookfield Engineering Laboratories, Stoughton, MA, 1985.
18. Allocock, H.R. and Lampe, F.W., Contemporary Polymer Chemistry, Prentice-Hall, Englewood Cliffs, NJ, 1981.
-
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Chapter 13
Mechanical Properties of Polymers
I.
INTRODUCTION
In Chapter 1, we observed that plastics have experienced a phenomenal growth since World War II, when
they assumed enhanced commercial importance. This explosive growth in polymer applications derives
from their competitive costs and versatile properties. Polymers vary from liquids and soft rubbers to
hard and rigid solids. The unique properties of polymers coupled with their light weight make them
preferable alternatives to metallic and ceramic materials in many applications. In the selection of a
polymer for a specific end use, careful consideration must be given to its mechanical properties. This
consideration is important not only in those applications where the mechanical properties play a primary
role, but also in other applications where other characteristics of the polymer such as electrical, optical,
or thermal properties are of crucial importance. In the latter cases, mechanical stability and durability
of the polymer may be required for the part to perform its function satisfactorily.
The mechanical behavior of a polymer is a function of its microstructure or morphology. Polymer
morphology itself depends on many structural and environmental factors. Compared with those of metals
and ceramics, polymer properties show a much stronger dependence on temperature and time. This strong
time and temperature sensitivity of polymer properties is a consequence of the viscoelastic nature of
polymers. This implies that polymers exhibit combined viscous and elastic behavior. For example, depending on the temperature and stress levels, a polymer may show linear elastic behavior, yield phenomena,
plastic deformation, or cold drawing. An amorphous polymer with Tg below ambient temperature may
display nonlinear but recoverable deformation or even exhibit viscous flow. Given the complexity of polymer
response to applied stresses or strains, it is imperative that, for their judicious use, those who work with
polymers have an elementary knowledge of how polymer behavior is influenced by structural and environmental factors. We have already dealt with the structures of polymers in earlier chapters. We devote
succeeding sections to discussing the mechanical properties of polymers in the solid state.
II.
MECHANICAL TESTS
Polymer components, like other materials, may fail to perform their intended functions in specific
applications as a result of
1. Excessive elastic deformation
2. Yielding or excessive plastic deformation
3. Fracture
Polymers show excessive elastic deformation, particularly in structural, load-bearing applications, due
to inadequate rigidity or stiffness. For such failure, the controlling material mechanical property is the
elastic modulus. As we shall see in subsequent discussions, the elastic moduli of some polymers are
subject to some measure of control through appropriate structural modification.
Failure of polymers in certain applications to carry design loads or occasional accidental overloads
may be due to excessive plastic deformation resulting from the inadequate strength properties of the
polymer. For the quantification of such failures, the mechanical property of primary interest is the yield
strength and the corresponding strain. The ultimate strength, along with the associated strain, also
provides useful information.
Cracks constitute regions of material discontinuity and frequently precipitate failure through fracture.
Fracture may occur in a sudden, brittle manner or through fatigue (progressive fracture). Brittle fracture
occurs in materials where the absence of local yielding results in a build-up of localized stresses, whereas
fatigue failure occurs when parts are subjected to alternating or repeated loads. Fatigue fractures occur
without visible signs of yielding since they occur at strengths well below the tensile strength of the
material.
0-8493-????-?/97/$0.00+$.50
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POLYMER SCIENCE AND TECHNOLOGY
Figure 13.1 Schematic of stress–strain test.
As we said earlier, polymers will continue to be used in a variety of end-use situations. Therefore,
to ensure their successful performance in these applications, it is necessary to clearly understand their
mechanical behavior under a variety of stress conditions. Particular cognizance must be taken of the
relatively high sensitivity of polymer failure modes to temperature, time, and loading history. For good
design, it is important to be able to relate design load and component dimensions to some appropriate
material property that defines the limits of the load-bearing capability of the polymer material. A variety
of test methods exist for predicting mechanical performance limits under a variety of loading conditions.
These range from simple tension, compression, and shear tests to those designed to test complex stress
states and polymer time–temperature response. Elaborate treatment of polymer deformation behavior
under complex stress states would require complex mathematical analysis, which is beyond the scope
of this volume. Our discussion emphasizes problems of a one-dimensional nature, and cases of nonlinear
deformation will be treated in an elementary fashion.
A. STRESS–STRAIN EXPERIMENTS
Stress–strain experiments have traditionally been the most widely used mechanical test but probably the
least understood in terms of interpretation. In stress–strain tests the specimen is deformed (pulled) at a
constant rate, and the stress required for this deformation is measured simultaneously (Figure 13.1). As
we shall see in subsequent discussions, polymers exhibit a wide variation of behavior in stress–strain
tests, ranging from hard and brittle to ductile, including yield and cold drawing. The utility of stress–strain
tests for design with polymeric materials can be greatly enhanced if tests are carried out over a wide
range of temperatures and strain rates.
B. CREEP EXPERIMENTS
In creep tests, a specimen is subjected to a constant load, and the strain is measured as a function of
time. The test specimen in a laboratory setup can be a plastic film or bar clamped at one end to a rigid
support while the load is applied suddenly at the other end (Figure 13.2). The elongation may be measured
at time intervals using a cathetometer or a traveling microscope. Measurements may be conducted in
an environmental chamber.
Creep tests are made mostly in tension, but creep experiments can also be done in shear, torsion,
flexure, or compression. Creep data provide important information for selecting a polymer that must
sustain dead loads for long periods. The parameter of interest to the engineer is compliance (J), which
is a time-dependent reciprocal of modulus. It is the ratio of the time-dependent strain to the applied
constant stress [J(t) = ε(t)/σ0]. Figure 13.3 shows creep curves for a typical polymeric material.
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MECHANICAL PROPERTIES OF POLYMERS
345
Figure 13.2 Schematic representation of creep experiment.
Figure 13.3 Creep of cellulose acetate at 45°C. (From Findley, W.N., Mod. Plast., 19(8), 71, 1942. With permission.)
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POLYMER SCIENCE AND TECHNOLOGY
Figure 13.4 Schematic of stress relaxation experiment.
C. STRESS RELAXATION EXPERIMENTS
In stress relaxation experiments, the specimen is rapidly (ideally, instantaneously) extended a given
amount, and the stress required to maintain this constant strain is measured as a function of time
(Figure 13.4). The stress that is required to maintain the strain constant decays with time. When this
stress is divided by the constant strain, the resultant ratio is the relaxation modulus (Er(t,T), which is a
function of both time and temperature. Figure 13.5 shows the stress relaxation curves for PMMA at
Figure 13.5 Log Er(t) vs. log t for unfractionated poly(methyl methacrylate). (From McLoughlin, J.R. and
Tobolsky, A.V., J. Colloid Sci., 7, 555, 1952. With permission.)
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MECHANICAL PROPERTIES OF POLYMERS
Figure 13.6 Torsion pendulum (A) is used to get data for typical response curve (B), indicating decreasing
amplitude of oscillation. (From Fried, J.R., Plast. Eng., 38(7), 27, 1982. With permission.)
different temperatures. Stress relaxation data provide useful information about the viscoelastic nature of
polymers.
D. DYNAMIC MECHANICAL EXPERIMENTS
In dynamic mechanical tests, the response of a material to periodic stress is measured. There are many
types of dynamic mechanical test instruments. Each has a limited frequency range, but it is generally
possible to cover frequencies from 10–5 to 106 cycles per second. A popular instrument for dynamic
mechanical measurements is the torsion pendulum (Figure 13.6A). A polymer sample is clamped at one
end, and the other end is attached to a disk that is free to oscillate. As a result of the damping
characteristics of the test sample, the amplitude of oscillation decays with time (Figure 13.6B).
Dynamic mechanical tests provide useful information about the viscoelastic nature of a polymer. It
is a versatile tool for studying the effects of molecular structure on polymer properties. It is a sensitive
test for studying glass transitions and secondary transitions in polymer and the morphology of crystalline
polymers.
Data from dynamic mechanical measurements can provide direct information about the elastic
modulus and the viscous response of a polymer. This can be illustrated by considering the response of
elastic and viscous materials to imposed sinusoidal strain, ε:
ε = ε 0 sin (ωt )
(13.1)
where ε0 is the amplitude and ω is the frequency (in radians per second, ω = 2πf; f is in cycles per
second). For a purely elastic body, Hooke’s law is obeyed. Consequently,
σ = Gε 0 sin (ωt )
(13.2)
where G is the shear modulus. It is evident from Equations 13.1 and 13.2 that for elastic bodies, stress
and strain are in phase.
Now consider a purely viscous fluid. Newton’s law dictates that the shear stress is given by σ = ηε· ,
that is,
σ = ηε 0 ω cos (ωt )
(13.3)
In this case, the shear stress and the strain are 90° out of phase. The response of viscoelastic materials
falls between these two extremes. It follows that the sinusoidal stress and strain for viscoelastic materials
are out of phase by an angle, say δ. The behavior of these classes of materials is illustrated in Figure 13.7.
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POLYMER SCIENCE AND TECHNOLOGY
Figure 13.7 The phase relation is shown between dynamic strain and stress for viscous, elastic, and viscoelastic
materials.
The lag angle between stress and strain is defined by the dissipation factor or tan δ given by
tan δ =
G ′′
G′
(13.4)
where G′ is the real part of the complex modulus (G″ = G′ + iG″), and G″ is the imaginary part of the
modulus. In physical terms, tan δ denotes material damping characteristics. It is a measure of the ratio
of the energy dissipated as heat to the maximum energy stored in the material during one cycle of
oscillation.
A convenient measure of damping is in terms of quantities determined from experiment. Thus,
tan δ =
1 A1
ln
π A2
(13.5)
where A1, and A2 are the amplitudes of two consecutive peaks (Figure 13.7). Alternatively, this may be
expressed in terms of log decrement (∆) for free vibration instruments like the torsional pendulum.
∆ = ln
A1
A
A
1
= ln 2 = ln i
A2
A3 n A i+n
(13.6)
It follows from Equations 13.5 and 13.6 that
∆ = π tan δ
(13.7)
E. IMPACT EXPERIMENTS
Polymers may also fail in service due to the effects of rapid stress loading (impact loads). Various test
methods have been proposed for assessing the ability of a polymeric material to withstand impact loads.
These include measurement of the area under the stress–strain curve in the high-speed (rapid) tensile
test; the measurement of the energy required to break a specimen by a ball of known weight released
from a predetermined height, the so-called falling ball or dart test; and the Izod and Charpy tests. The
most popular of these tests methods are the Izod and Charpy impact strength tests. Essentially, the Izod
test involves the measurement of the energy required to break a H × H in. notched cantilever specimen
that is clamped rigidly at one end and then struck at the other end by a pendulum weight. In the case
of the Charpy test, a hammerlike weight strikes a notched specimen that is rigidly held at both ends.
The energy required to break the specimen is obtained from the loss in kinetic energy of the hammer
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349
MECHANICAL PROPERTIES OF POLYMERS
Figure 13.8 Schematic representation of impact test. (From Fried, J.R., Plast. Eng., 38(7), 27, 1982. With
permission.)
(Figure 13.8). Another test that is emerging as a substitute for the impact test is the measurement of
fracture toughness, which in essence measures the resistance to failure of a material with a predetermined
crack.
Impact tests provide useful information in the selection of a polymer for a specific application, such
as determining the suitability of a given plastic as a substitute for glass bottles or a replacement for
window glass. Table 13.1 gives values of impact energies for some polymers. It can be seen that, in
Table 13.1 Impact Energies of Some Polymers
Polymer
Polystyrene
Polystyrene
Poly(vinyl chloride)
Poly(vinyl chloride)
Polypropylene
Poly(methyl methacrylate)
Poly(methylmethacrylate)
Polyoxymethylene
Nylon 6,6
Nylon 6
Poly(propyleneoxide)
Polycarbonate
Polyethylene
Polyethylene
Polytetrafluoroethylene
Polypropylene
Copyright 2000 by CRC Press LLC
Grade
Impact Energy (J)
General purpose
Impact
Rapid
Plasticized
Unmodified
Molding
High impact
0.34–0.54
0.68–10.80
0.54–4.07
1.36–20.33
0.68–2.71
0.41–0.68
1.90
1.90–3.12
1.36–3.39
1.36–4.07
6.78
16.26–24.39
21.70
0.68–27.10
4.07
0.68–2.71
Low density
High density
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POLYMER SCIENCE AND TECHNOLOGY
Figure 13.9 Typical tensile specimen.
general, brittle polymers like general-purpose polystyrene have low impact resistance, while most
engineering thermoplastics like polyamides, polycarbonate, and polyoxymethylene and some commodity
thermoplastics like polyethylene, polypropylene, and polytetrafluoroethylene have high impact strength.
III.
STRESS–STRAIN BEHAVIORS OF POLYMERS
In stress–strain experiments, a polymer sample is pulled (deformed) at a constant elongation rate, and
stress is measured as a function of time. Figure 13.9 shows a typical tensile specimen (ASTM D638M).
Generally the polymer specimen, which may be rectangular or circular in cross-section, is molded or
cut in the form of a dog bone. It is clamped at both ends and pulled at one of the clamped ends (usually
downward) at constant elongation. The shape of the test specimen is designed to encourage failure at
the thinner middle portion. The central section between clamps is called the initial gauge length, L0.
The load or stress is measured at the fixed end by means of a load transducer as a function of the
elongation, which is measured by means of mechanical, optical, or electronic strain gauges. The experimental data are generally stated as engineering (nominal) stress (σ) vs. engineering (nominal) strain
(ε). The engineering stress is defined as
σ=
F
A0
(13.8)
where F = the applied load
A0 = the original cross-sectional area over the specimen
The engineering strain is given by
ε=
L − L 0 ∆L
=
L0
L0
(13.9)
where L0 = original gauge length
∆L = elongation or change in the gauge length
L = instantaneous gauge length
Engineering stress and strain are easy to calculate and are used widely in engineering practice. However,
engineering stress–strain curves generally depend on the shape of the specimen. A more accurate measure
of intrinsic material performance is plots of true stress vs. true strain. True stress σt is defined as the
ratio of the measured force (F) to the instantaneous cross-sectional area (A) at a given elongation, that is,
σt = F A
(13.10)
True strain is the sum of all the instantaneous length changes, dL, divided by the instantaneous length L.
εt =
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∫
L
L0
dL
L
= ln
L
L0
(13.11)
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MECHANICAL PROPERTIES OF POLYMERS
Some relation exists between true strain and engineering strain and between true stress and engineering
stress. From Equation 13.11,
ε t = ln
L + ∆L
L
= ln 0
= ln (1 + ε)
L0
L0
(13.12)
Up to the onset of necking, plastic deformation is essentially a constant volume process such that any
extension of the original gauge length is accompanied by a corresponding contraction of the gauge
diameter. Thus,
AL = A 0 L 0
(13.13)
A
L
= 0
L0
A
(13.14)
That is,
But from Equation 13.12 it follows that
ε t = ln
A0
A
and 0 = 1 + ε
A
A
(13.15)
Now
σt =
F
F A0
=
⋅
A A0 A
That is,
σ t = σ (1 + ε)
(13.16)
For small deformations, true stress and engineering stress are essentially equal. However, for large
deformations the use of true strain is preferred because they are generally additive while engineering
strain is not.
A. ELASTIC STRESS–STRAIN RELATIONS
When a material is subjected to small stresses, it responds elastically. This means that
1. The strain produced is reversible with stress.
2. The magnitude of the strain is directly or linearly proportional to the magnitude of the stress for material
that exhibits Hookean behavior. This relation between stress and strain is known as Hooke’s law and
may be written as
Stress
= Constant
Strain
(13.17)
Since stress may act on a plane in different ways, this constant is defined in different ways depending
on the applied force and the resultant strain. Two of the most important types of stress are shear stress,
which acts in a plane, and tensile stress, which acts normally or perpendicular to the plane. Normal
stresses may be tensile or compressive.
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POLYMER SCIENCE AND TECHNOLOGY
Figure 13.10 Generation of shear strain from simple shear.
Figure 13.11 Pure dilatation.
Consider a parallelepiped that has been deformed by application of a force τ (Figure 13.10). The
elastic shear strain γ is the amount of their ∆X divided by the distance, h, over which the shear has
occurred:
γ=
∆x
h
(13.18)
For small strains, this is simply the tangent of the angle of deformation. In pure shear, Hooke’s law is
expressed as
τ γ =G
(13.19)
where τ is the shear stress and G is the shear modulus. Deformation due to pure shear does not result
in a change in volume, but produces a change in shape.
Now suppose the parallelepiped is subjected to equal normal pressure (compressive stress, –σ) in such
a way that its shape remains unchanged but the volume, V, is changed by the amount ∆V (Figure 13.11).
Deformation of this type is called pure dilatation, and Hooke’s law for elastic dilatation is written as
σ = KD
(13.20)
where K is the bulk modulus and D is the dilatation strain given by ∆V/V. We reemphasize that pure
dilatation does not produce a change in shape but produces a change in volume.
In a majority of cases, a body under stress experiences neither pure shear nor pure dilatation. Generally,
a mixture of both occurs. Such a situation is exemplified by uniaxial loading which, of course, may be
tensile or compressive. Here a test specimen is loaded axially resulting in a change in length, ∆L. The
axial strain, ε, is related to the applied stress in an elastic deformation by Hooke’s law:
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353
MECHANICAL PROPERTIES OF POLYMERS
Figure 13.12 Axial elongation accompanied by transverse contractions.
σ = Eε
(13.21)
where E is Young’s modulus (modulus of elasticity).
During extension, specimen elongation in the axial or longitudinal direction is accompanied by a
contraction in the perpendicular transverse directions, given by the compressive strains εy = εz as shown
in Figure 13.12. Poisson’s ratio, denoted by the parameter ν, is the ratio of the induced transverse strains
to the axial strain.
ν=−
εy
εx
=−
εz
εx
(13.22)
The negative sign indicates that the strains εz and εy are due to contractions.
For incompressible materials, the volume of the specimen remains constant during deformation, and
ν is 0.5. This is generally not true, although it is approached by natural rubber with ν = 0.49. For most
polymeric materials, there is a change in volume ∆V, which is related to Poisson’s ratio by
∆V = (1 − 2 ν) εV0
(13.23)
or, in general,
ν=
1
1 ∂V
1−
2 V ∂ε
where V0 is the initial (unstrained) volume and ∆V is the difference between the volume V at a given
strain ε and the initial volume.
For materials that are isotropic and under deformations where Hooke’s law is valid, the elastic
constants and ν are related according to the following equations:
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E = 2G (1 + ν)
(13.24)
E = 3K (1 − 2ν)
(13.25)
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POLYMER SCIENCE AND TECHNOLOGY
Example 13.1: In a tension test, a brittle polymer experienced an elastic engineering strain of 2% at a
stress level of 35 MN/m2. Calculate
a. The true stress
b. The true strain
c. The fractional change in cross-sectional area
Solution: Under elastic conditions, deformation is a constant-volume process, hence:
σ t = σ (1 + ε)
ε = 0.02
σ t = 35 (1 + 0.02) MN m 2
= 35.7 MN m 2
ε t = ln (1 + ε)
= ln (1 + 0.02)
= 0.0198
A0 − A
A
1
A
since 0 = 1 + ε
= 1−
= 1−
1+ ε
A0
A0
A
= 1−
1
1.02
= 0.0196
Example 13.2: Polypropylene has an elastic modulus of 2 × 105 psi and Poisson’s ratio of 0.32. For a
strain of 0.05, calculate the shear stress and the percentage change in volume.
Solution:
E = 2G (1 + ν)
G=
E
2 (1 + ν)
τ = Gγ =
=
E
γ
2 (1 + ν)
2 × 10 5
× 0.05
2 (1 + 0.32)
= 3.79 × 103 psi
∆V = (1 − 2 ν) γV0
∆V
= (1 − 2 ν) γ
V0
= [1 − 2 (0.32)] 0.05
= 1.8%
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MECHANICAL PROPERTIES OF POLYMERS
355
Figure 13.13 Engineering data from stress–strain tests.
IV.
DEFORMATION OF SOLID POLYMERS
To relieve stress, all materials under the influence of external load undergo some deformation. In some
cases, this deformation may be quite large and perceptible, like the boughs of a mango tree under the
weight of its fruit. In some other cases, deformation may be imperceptibly small, for example, a fly
perching uninvitedly on the dining table. Irrespective of our ability to observe it, some deformation
always occurs when stresses are imposed on a material.
It is known that up to a certain limiting load, a solid will recover its original dimensions on the
removal of the applied loads. This ability of deformed bodies to recover their original dimensions is
known as elastic behavior. Beyond the limit of elastic behavior (elastic limit), a material will experience
a permanent set or deformation even when the load is removed. Such a material is said to have undergone
plastic deformation. For most materials, Hooke’s law is obeyed within the elastic limit, that is, stress
proportional to strain. However, proportionality between stress and strain does not always hold when a
material exhibits elastic behavior. A typical example is rubber, which is elastic but does not show Hookean
behavior over the entire elastic region.
Figure 13.13 illustrates the basic data on mechanical properties that are obtainable from stress–strain
experiments. The gradient of the initial linear portion of the curve, within which Hooke’s law is obeyed,
gives the elastic, or Young’s, modulus. The determination of the elastic limit is tedious and very frequently
depends on the sensitivity of the strain-measuring devices employed. Consequently, it is common practice
to replace it with the proportional limit, which defines the point where the nonlinear response is observed
on the stress–strain curve. The maximum on the curve denotes the yield strength. For engineering
purposes, this marks the limit of usable elastic behavior or the onset of plastic deformation. The stress
at which fracture occurs (material breaks apart) is referred to as the ultimate tensile strength or, simply,
tensile strength σB. The strains associated with the yield point or the fracture point are referred to as the
elongation at yield and elongation at break, respectively. Typical values of these mechanical properties
for selected polymers are shown in Table 13.2.
To emphasize the usefulness of stress-strain measurements, it is necessary to highlight the physical
significance of the parameters defined above or the mechanical quantities derivable from these parameters:4
• Stiffness — Defines the ability to carry stress without changing dimension. The magnitude of the
modulus of elasticity is a measure of this ability or property.
• Elasticity — Stipulates the ability to undergo reversible deformation or carry stress without suffering
a permanent deformation. It is indicated by the elastic limit or, from a practical point of view, the
proportional limit or yield point.
• Resilience — Defines the ability to absorb energy without suffering permanent deformation. The area
under the elastic portion of the stress-strain curve gives the resilient energy.
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POLYMER SCIENCE AND TECHNOLOGY
Table 13.2 Typical Mechanical Properties of Selected Polymers
Polymer
Polypropylene
Polystyrene
Poly(methylmethacrylate)
Polyethylene (LDPE)
Polycarbonate
Poly(vinyl chloride (PVC), rigid
Polytetrafluoroethylene
Poisson
Ratio
Elastic
Modulus
(103 psi)
Yield
Strength
(103 psi)
Ultimate
Strength
(103 psi)
Elongation
to Fracture
(%)
0.32
0.33
0.33
0.38
0.37
0.40
0.45
1.5–2.25
4–5
3.5–5
0.2–0.4
3.5
3–6
0.6
3.4
—
7–9
1–2
8–10
8–10
1.5–2
3.5–5.5
5.5–8
7–10
1.5–2.5
8–10
6–11
2–4
200–600
1–2.5
2–10
400–700
60–120
5–60
100–350
From Fried, J.R., Plast. Eng., 38(7), 27, 1982. With permission.
• Strength — Indicates the ability to sustain dead load. It is represented by the tensile strength or the
stress at which the specimen ruptures σB.
• Toughness — Indicates the ability to absorb energy and undergo extensive plastic deformation without
rupturing. It is measured by the area under the stress-strain curve.
The response of material to applied stress may be described as ductile or brittle depending on the
extent to which the material undergoes plastic deformation before fracture. Ductile materials possess the
ability to undergo plastic deformation. For engineering purposes, an appropriate measure of ductility is
important, because this property assists in the redistribution of localized stresses. On the other hand, brittle
materials fail with little or no plastic deformation. Brittle materials have no ability for local yielding;
hence local stresses build up around inherent flaws, reaching a critical level at which abrupt failure occurs.
Figure 13.14 shows the broad spectrum of stress-strain behavior of polymeric materials, while Table 13.3
lists the general trends in the magnitude of various mechanical parameters typical of each behavior.
From the preceding discussion (Figure 13.14 and Table 13.3), it is obvious that polymers have a broad
range of tensile properties. It is therefore instructive to examine these properties more closely and present
current molecular interpretation at the observed properties. Figure 13.15 is a schematic representation
of the macroscopic changes that occur in polymers that exhibit cold drawing during a tensile test.
At small strains, polymers (both amorphous and crystalline) show essentially linear elastic behavior.
The strain observed in this phase arises from bond angle deformation and bond stretching; it is recoverable
on removing the applied stress. The slope of this initial portion of the stress–strain curve is the elastic
modulus. With further increase in strain, strain-induced softening occurs, resulting in a reduction of the
instantaneous modulus (i.e., slope decreases). Strain-softening phenomenon is attributed to uncoiling
Figure 13.14 Typical stress–strain curves for polymeric materials. (From Winding, C.C. and Hiatt, G.D., Polymer
Materials, McGraw-Hill, New York, 1961. With permission.)
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MECHANICAL PROPERTIES OF POLYMERS
Table 13.3 Characteristic Features of Polymer Stress–Strain Behavior
Elastic
Modulus
Yield
Point
Soft and weak (polymer gels)
Hard and brittle (PS)
Low
High
Hard and strong (PVC)
Soft and tough (rubbers and plasticized PVC)
Hard and tough (cellulose acetate, nylon)
High
Low
High
Low
Practically
nonexistent
High
Low
High
Material Stress-Strain Behavior
Tensile
Strength
Elongation
at Break
Low
High
Moderate
Low
High
Moderate
High
Moderate
High
High
Figure 13.15 Schematic representation of macroscopic changes in tensile specimen shape during cold drawing.
(From Fried, J.R., Plast. Eng., 38(7), 27, 1982. With permission.)
and straightening of polymer chains, and the associated strain is recoverable. For hard, rigid polymers
like polystyrene that show little or no yielding, a further increase in stress results in brittle failure
(curve 1). In the case of ductile polymers, including engineering thermoplastics (polyamides, high-impact
polystyrene), the stress–strain curve exhibits a maximum: the stress reaches a maximum value called
the yield stress (more precisely, upper yield stress) and then decreases to a minimum value (drawing
stress or lower yield stress). At this point, the sample may either rupture or experience strain hardening
before failure. At the drawing stress, polymers that strain-harden require no further increase in applied
stress to induce further elongation. It is believed that at the yield stress some slippage of polymer chains
past each other occurs. The attendant deformation is recovered partially and slowly on the removal of
the applied stress. What happens after the upper yield stress depends on the ability of the polymer
material to strain-harden. The onset of necking is associated with an increase in the local stress at the
necked region due to the reduction in the load-bearing cross-sectional area. This results in extensive
deformation of the polymer material in the vicinity of the necked region, and the polymer chains in the
amorphous regions undergo conformational changes and become oriented (stretched) in the direction of
the applied tensile stress. The extended chains resist further deformation. If this orientation-induced
hardening or resistance is sufficiently high to sustain or overcome the increased stress due to the reduction
in the cross-sectional area (following the onset of necking), then further deformation (extension) of the
specimen will occur only through the propagation of the neck along the sample. On the other hand, if
the increased stress at the neck region increases faster than orientation hardening, then the necked region
deepens continuously, leading ultimately to local failure at that region.
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POLYMER SCIENCE AND TECHNOLOGY
The molecular orientation of polymer chains is reflected in the observed changes of shape of the
specimen (curve 3, Figure 13.15). Up to the yield stress, specimen deformation is essentially homogeneous. This means that deformation occurs uniformly over the entire gauge length of the specimen. At
the yield stress, local instability ensues and the specimen begins to neck at some point along its gauge
length. For specimens that exhibit orientation hardening before failure, the neck stabilizes; that is, the
specimen shows no further reduction in cross-sectional area, but the neck propagates along the length
of the gauge section until the specimen finally ruptures. The process of neck propagation is referred to
as cold drawing.
Example 13.3: The mechanical properties of nylon 6,6 vary with its moisture content. A nylon specimen
with a moisture content (MC) of 2.5% has an elastic modulus of 1.2 GN/m2, while that for a sample of
moisture content of 0.2% is 2.8 GN/m2. Calculate the elastic energy or work per unit volume in each
sample subjected to a tensile strain of 10%.
Solution:
Work = [Force, (F)] [increment of extension, (dL)]
Work per unit volume =
∫
FdL
=
AL
ε
σdε
0
In the elastic region, Hooke’s law holds, i.e., σ = Eε.
Work per volume =
∫
ε
Eεdε =
0
Eε 2
2
Sample 1 (2.5% MC):
Elastic energy =
(1.2 × 10
9
)
N m 2 (0.1)
2
2
= 0.6 × 10 9 × 10 −2 N - m m 3
= 6 MJ m 3
Sample 2 (0.2% MC):
Elastic energy =
= 14 MJ m
(2.8 × 10
9
)
N m 2 (0.1)
2
2
3
Example 13.4: Two nylon samples of moisture contents 2.5% and 0.2% have εB of 300% and 60%,
respectively. Calculate the toughness of each sample if the stress-strain curve of nylon for plastic
deformation is given by
σ = 8500 ε0.1 psi
Comment on your results.
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MECHANICAL PROPERTIES OF POLYMERS
Solution:
Work per unit volum (toughness) =
=
∫
εB
σdε
0
∫
εB
8500ε 0.1dε
0
= 7727 ε1.1 in.- lb in.3
Nylon 2.5% MC, toughness
= 7727 (3)
1.1
= 25, 873 in.- lb in.3
Nylon 0.2% MC, toughness
= 7727 (0.6)
1.1
= 4405 in.- lb in.3
As MC increased from 0.2 to 2.5%, toughness increased by about 500%.
V. COMPRESSION VS. TENSILE TESTS6
Recall that normal stresses can be either tensile or compressive. However, the main focus of our
discussion so far has been on tensile tests. Let us now examine the behavior of polymers in compression.
Figures 13.16 and 13.17 are plots of the compressive stress–strain data for two amorphous and two
crystalline polymers, respectively, while Figure 13.18 shows tensile and compressive stress–strain behavior of a normally brittle polymer (polystyrene). The stress–strain curves for the amorphous polymers
are characteristic of the yield behavior of polymers. On the other hand, there are no clearly defined yield
points for the crystalline polymers. In tension, polystyrene exhibited brittle failure, whereas in compression it behaved as a ductile polymer. The behavior of polystyrene typifies the general behavior of
polymers. Tensile and compressive tests do not, as would normally be expected, give the same results.
Strength and yield stress are generally higher in compression than in tension.
Figure 13.16 Compressive stress–strain data for two amorphous polymers: polyvinyl chloride (PVC) and
cellulose acetate (CA). (From Kaufman, H.S. and Falcetta, J.J., Eds., Introduction to Polymer Science and
Technology, John Wiley & Sons, New York, 1977. With permission.)
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Figure 13.17 Compressive stress–strain data for two crystalline polymers: polytetrafluoroethylene (PTFE) and
polychlorotrifluoroethylene (PCTFE). (From Kaufman, H.S. and Falcetta, J.J., Eds., Introduction to Polymer
Science and Technology, John Wiley & Sons, New York, 1977. With permission.)
Figure 13.18 The stress–strain behavior of a normally brittle polymer, polystyrene, under tension and compression. (From Nielsen, L.E., Mechanical Properties of Polymers and Composites, Vol. 2, Marcel Dekker, New
York, 1974. With permission.)
The tensile properties of brittle materials depend to a considerable extent on the cracks and other
flaws inherent in the material. As we shall see later, for a material in tension, brittle fracture occurs
through the propagation of one of these cracks. Since load cannot be transmitted through free surfaces,
the presence of cracks essentially creates concentrations of stress intensity. When this tensile stress
exceeds the fracture strength of the material, fracture occurs. It is apparent, therefore, that in contrast
to tensile stresses, which open cracks, compressive stresses tend to close them. This could conceivably
enhance the tensile strength.
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MECHANICAL PROPERTIES OF POLYMERS
Figure 13.19 Generation of shear stress due to uniaxial loading.
The conditions for fracture are dictated by the magnitude of the imposed tensile stresses. On the
other hand, the extent of permanent (plastic) deformation produced by a given load depends primarily
on the magnitude of the shear stresses induced by the load. For example, the application of a uniaxial
load, F, on a material generates shear stresses, τ, in certain geometric planes in particular directions.
The magnitude of the generated shear stresses depends on the orientations of the planes and directions
to the tensile axis. This can be illustrated by considering Figure 13.19.
Let us compute the shear stresses generated on the sectional plane A′ in a direction of angle θ to the
tensile axis.8 The normal to plane A′ makes an angle φ with the tensile axis. The load on A′ in the
direction of deformation (slip) is F cos θ. Therefore, the stress generated on A′ is given by
τ=
F cos θ
A cos φ
(13.26)
But by definition, F/A = σ, hence
τ = σ cos θ cos φ
(13.27)
For a fixed value of φ, the minimum value of θ is π/2 – φ. Thus Equation 13.27 becomes
π
τ = σ cos φ cos − φ = σ cos φ sin φ
2
(13.28)
The maximum value of τ occurs when φ = π/4, that is,
(τ) max = σ (0.707)2 =
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σ
2
(13.29)
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POLYMER SCIENCE AND TECHNOLOGY
Plastic deformation occurs when τmax is at least equal to the yield strength of the material, i.e., for
yielding to occur.
τ max ≥ σ y
(13.30)
The yield criterion for simple uniaxial stress dictates theoretically that for plastic deformation to occur,
the imposed tensile stress must be at least twice the magnitude of the shear stresses generated. In other
words, the tensile strength must be at least twice the shear strength. This is often not the case in practice;
tensile strength is generally less than twice the shear strength. Also, the fact that contrary to theoretical
prediction, the yield value for a given material is not the same in tension and in compression suggests
that for polymers, plastic deformation may not be due entirely to shear stresses alone.
VI.
EFFECTS OF STRUCTURAL AND ENVIRONMENTAL FACTORS
ON MECHANICAL PROPERTIES
As we saw in the preceding discussion, several mechanical parameters can be derived from stress-strain
tests. Two of these parameters are of particular significance from a design viewpoint. These are strength
and stiffness. For some applications, the ultimate tensile strength is the useful parameter, but most
polymer products are loaded well below their breaking points. Indeed, some polymers deform excessively
before rupture and this makes them unsuitable for use. Therefore, for most polymer applications, stiffness
(resistance to deformation under applied load) is the parameter of prime importance. Modulus is a
measure of stiffness. We will now consider how various structural and environmental factors affect
modulus in particular and other mechanical properties in general.
A. EFFECT OF MOLECULAR WEIGHT
In contrast to materials like metals and ceramics, the modulus of polymers shows strong dependence
on temperature. Figure 13.20 is a schematic modulus–temperature curve for a linear amorphous polymer
like atactic polystyrene. Five regions of viscoelastic behavior are evident: a hard and glassy region
followed by a transition from the glassy to rubbery region. The rubbery plateau, in turn, is followed by
a transition to the melt flow region. The glassy-to-rubbery transition is denoted by Tg, while the rubberto-melt-flow transition is indicated by Tfl. There is a drop in modulus of about three orders of magnitude
near the Tg. There is a further modulus drop at Tfl. If the Tg is above room temperature, the material
will be a rigid polymer at room temperature. If, however, the Tg occurs below room temperature, the
material will be rubbery and might even be a viscous liquid at room temperature.
Figure 13.20 Schematic representation of the effect of molecular weight on shear modulus–temperature curve.
Tg is the glass transition temperature while Tfl is the flow temperature. Tfl′, Tfl″, Tfl- represent low-, medium-, and
high-molecular-weight materials, respectively.
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MECHANICAL PROPERTIES OF POLYMERS
363
Figure 13.21 Effect of cross-linking on shear modulus of natural rubber: (———) cross-linked, the approximate
mean number of chain atoms between successive cross-links is indicated; (– – – –) noncross-linked. (From
Heijboer, J., Br. Polym. J., 1, 3, 1969. With permission.)
The molecular weight has practically no effect on the modulus in the glassy region (below Tg); the
drop in modulus and the location of the Tg are also almost independent of the molecular weight. We
recall that Tg is the onset of cooperative motion of chain segments. It is therefore to be expected that
the number of chain entanglements (which increases with increasing molecular weight) will hardly affect
such segmental mobility, and consequently Tg is independent of molecular weight. In contrast to the Tg,
Tfl is strongly dependent on the molecular weight. Movement of entire molecules is associated with
viscous flow. This obviously will depend on the number of entanglements. The higher the molecular
weight (the higher the number of entanglements), the higher must be the temperature at which viscous
flow becomes predominant over rubbery behavior. Consequently, for high molecular weight polymer,
Tfl is high and the rubbery plateau long, whereas for low-molecular-weight polymers the rubbery plateau
is absent or very short.
B. EFFECT OF CROSS-LINKING
Figure 13.21 shows the effect of cross-linking on the modulus of natural rubber cross-linked using
electron irradiation. Mc is the average molecular weight between cross-links. It is a measure of the crosslink density; the smaller the value of Mc, the higher the cross-link density. In the glassy region, the
increase in modulus due to cross-linking is relatively small. Evidently the principal effect of cross-linking
is the increase in modulus in the rubbery region and the disappearance of the flow regions. The crosslinked elastomer exhibits rubberlike elasticity even at high temperature. Cross-linking also raises the
glass transition temperature at high values of cross-link density. The glass-to-rubber transition is also
considerably broadened.
Cross-linking involves chemically connecting polymer molecules by primary valence bonds. This
imposes obvious restrictions on molecular mobility. Consequently, relative to the uncross-linked polymer,
cross-linking increases polymer ability to resist deformation under load, i.e., increases its modulus. As
would be expected, this effect is more pronounced in the rubbery region. In addition, the flow region is
eliminated in a cross-linked polymer because chains are unable to slip past each other. Since Tg represents
the onset of cooperative segmental motion, widely spaced cross-links will produce only a slight restriction
on this motion. However, as the cross-link density is increased, the restriction on molecular mobility
becomes substantial and much higher energy will be required to induce segmental motion (Tg increases).
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POLYMER SCIENCE AND TECHNOLOGY
Example 13.5: The densities of hard and soft rubbers are 1.19 and 0.90 g/cm3, respectively. Estimate
the average molecular weight between cross-links for both materials if their respective moduli at room
temperature are 106 and 108 dynes/cm2.
Solution: The average molecular weight between cross-links, Mc is given by
Mc = ρ
RT
G
where ρ = density, R = gas constant, T = absolute temperature, and G = shear modulus.
(1.19 g cm )(8.31 × 10 erg mol K)(300 K)
(10 dyne cm )
3
Mc =
7
8
2
g erg cm 3
= 300
mol cm 2 dyne
g erg cm 3
= 300
mol cm 3 dyne - cm
= 300 g mol
Soft rubber:
(0.90 g cm ) (8.31 × 10
3
Mc =
7
7
)
erg mol K (300 K)
10 dyne cm
2
= 2250 g mol
C. EFFECT OF CRYSTALLINITY
Crystallinity in a polymer is the result of ordered molecular aggregation, with molecules held together
by secondary valence bonds. Therefore, crystallinity may be viewed as a form of physical cross-linking
which is thermoreversible. Since crystallites have much higher moduli than the amorphous segments,
crystallites can also be regarded as rigid fillers in an amorphous matrix. The effect of crystallinity on
modulus becomes readily understandable on the basis of these concepts. Figure 13.22 is an idealized
modulus–temperature curve for various degrees of crystallinity. We observe that crystallinity has only a
small effect on modulus below the Tg but has a pronounced effect above the Tg. There is a drop in modulus
at the Tg, the intensity of which decreases with increasing degree of crystallinity. This is followed by a
much sharper drop at the melting point. Crystallinity has no significant effect on the location of the Tg,
but the melting temperature generally increases with increasing degree of crystallinity. These features
are evident for two polyethylenes of different crystallinities (Figure 13.23). Alkathene is branched with
a density of 0.92 g/cm3, while the Ziegler polyethylene is linear and has a density of 0.95 g/cm3. The
greater crystallinity of the Ziegler polyethylene results in a higher modulus, especially above 0°C.
D. EFFECT OF COPOLYMERIZATION
In discussing the effect of copolymerization on modulus, it is necessary to make a distinction between
random and alternating copolymers and block and graft comonomers. Random and alternating copolymers are necessarily homogeneous, while block and graft copolymers with sufficiently long sequences
exhibit phase separation. Random and alternating copolymers show a single transition that is intermediate
between those of the two homopolymers of A and B (Figure 13.24). Copolymerization essentially shifts
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MECHANICAL PROPERTIES OF POLYMERS
365
Figure 13.22 Effect of crystallinity on the modulus–temperature curve. The numbers of the curves are rough
approximations of the percentage of crystallinity. Modulus units = dynes/cm2. (From Nielsen, L.E., Mechanical
Properties of Polymers and Composites, Vol. 2, Marcel Dekker, New York, 1974. With permission.)
Figure 13.23 Shear modulus (a) and damping (b) at I Hz as a function of temperature for a branched and a
linear polyethylene: (– – – –) Ziegler polyethylene; (———) Alkathene. (From Heijboer, J., Brit. Polymer J., 1, 3,
1969. With permission.)
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POLYMER SCIENCE AND TECHNOLOGY
Figure 13.24 Effect of plasticization or copolymerization on the modulus–temperature curve. The curves correspond to different copolymer compositions. (B) Unplasticized homopolymer; (A) either a second homopolymer
or plasticized B. (From Nielsen, L.E., Mechanical Properties of Polymers and Composites, Vol. 2, Marcel Dekker,
New York, 1974. With permission.)
the modulus–temperature curve the same way as Tg. In addition, there is a broadening of the transition
due to polymer composition heterogeneity. We recall that in copolymer the reactivities of the monomers
are generally different. Consequently, the initial polymer formed is richer in the more reactive monomer
while that formed later is richer in the less reactive monomer. This leads overall to a polymer of
heterogeneous composition and consequently a distribution of glass transitions (broad transition region).
Block and graft copolymers, which exist as a two-phase system, have two distinct glass transitions,
one for each of the homopolymers. Consequently, the modulus–temperature curve shows two steep
drops. The value of modulus in the plateau between the two glass transitions depends upon the ratio of
the components and upon which of the two phases is dispersed in the other. These features are illustrated
in Figure 13.25 for a styrene–butadiene–styrene block copolymer. The glass transition of the butadiene
phase near –80°C and that for the styrene phase near 110°C are clearly evident. Between the Tg of
butadiene and the Tg of styrene, the value of the modulus is determined by the amount of polystyrene;
the rubbery butadiene phase is cross-linked physically by the hard and glassy polystyrene phase. It is
noteworthy that while styrene–butadiene–styrene block copolymers have high tensile strength, butadiene–styrene–butadiene copolymers have a very low tensile strength, showing that strength properties are
dictated by the dispersed phase.
In both cases of copolymerization, there is a noticeable decrease in the slope of the modulus curve
in the region of the inflection point. This, in essence, means a decrease in the modulus in the rubbery
region. This contrasts with the chemically cross-linked systems where the modulus in the rubbery region
shows some increase with increasing temperatures. In the copolymer system, the molecules are interconnected by physical cross-links due to secondary forces. These cross-links can be disrupted reversibly
by heating, and this forms the basis of the new class of copolymers referred to as thermoplastic elastomers.
E. EFFECT OF PLASTICIZERS
Plasticizers are low-molecular-weight, usually high boiling liquids that are capable of enhancing the
flow characteristics of polymers by lowering their glass transition temperatures. Modulus, yield, and
tensile strengths generally decrease with the addition of plasticizers to a polymer. In general, on
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MECHANICAL PROPERTIES OF POLYMERS
Figure 13.25 Shear modulus as a function of temperature for styrene–butadiene–styrene block copolymers.
Wt.% styrene is indicated on each curve. (From Heijboer, J., Br. Polymer J., 1, 3, 1969. With permission.)
plasticization a polymer solid undergoes a change from hard and brittle to hard and tough to soft and
tough. This is exemplified by the use of dioctylphthalate to convert poly(vinyl chloride) from a rigid
material, such as PVC pipes, to a soft one, as in car seat covers or a raincoat. Plasticization and alternating
or random copolymerization have similar effects on modulus (Figure 13.24). In this case B is the
unplasticized homopolymer, while curves 1, 2 and 3 represent increasing plasticization of B.
F. EFFECT OF POLARITY
The effect of polarity is shown in Figure 13.26, which compares poly(vinyl chloride) with polypropylene.
The Tg of the polar poly(vinyl chloride) is about 90°C higher than that of the nonpolar polar polypropylene. As the methyl group and chlorine atom occupy about the same volume, the differences in
mechanical behavior can only be due to the relative polarities of the two polymers. However, the effect
of the substitution of the chlorine atom for the methyl group depends on the molecular environment of
the chlorine atom. The further the chlorine atom is from the main chain, the smaller its effect on the Tg.
This is illustrated in Figure 13.27, which compares the mechanical behavior of the following polymers:
Polymer
Structure
Poly(2-chloroethyl methacrylate)
CH3
CH2
C
C
O
CH2
CH2
Cl
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O
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POLYMER SCIENCE AND TECHNOLOGY
Polymer
Structure
Poly(n-propyl methacrylate)
CH3
CH2
C
C
O
O
CH2
CH2
CH3
Figure 13.26 Shear modulus (a) and damping (b) as a function of temperature: (———) poly(vinyl chloride);
(– – – –) polypropylene. (From Heijboer, J., Br. Polym. J., 1, 3, 1969. With permission.)
In this case, the Tg of poly(2-chloroethyl methacrylate) is only 20°C higher than that of poly(n-propyl
methacrylate). However, with the secondary transitions, which represent the movement of the side chains
(chloroethyl and n-propyl groups, respectively), the effect on the location of the glass temperature is
much larger. Also, the modulus of the polar polymer is much higher. In the glassy region, the magnitude
of the modulus is determined primarily by secondary bonding forces. It is therefore to be expected that
the modulus will be increased by the introduction of polar forces. Indeed, polarity is generally the most
effective means for increasing modulus in the glassy region.
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MECHANICAL PROPERTIES OF POLYMERS
Figure 13.27 Shear modulus (a) and damping (b) at 1 Hz as a function of temperature: (———) poly(2chloroethyl methacrylate); (– – – –) poly(n-propyl methacrylate. (From Heijboer, J., Br. Polym. J., 1, 3, 1969. With
permission.)
Example 13.6: Both polyoxymethylene (DuPont Delrin 550) and polyethylene (Calanese Fortiflex A70)
show similar mechanical behavior but the glass transition temperature of polyoxymethylene is about
50°C higher than that of polyethylene. Explain.
Solution:
Polymer
Structure
Polyoxymethylene
Polyethylene
–CH2–O–
–CH2–CH2–
The presence of the oxygen atom in the main chain of polyoxymethylene might have been expected to
enhance its flexibility compared with polyethylene and hence reduce its Tg relative to that of polyethylene.
However, we note that the dipole character of the C–O–C group produces polar forces between adjacent
chains, which act over a longer range and are stronger than van der Waals forces. Thus, for polyoxymethylene the induced flexibility is more than offset by the increased bonding forces resulting from
polarity.
G. STERIC FACTORS
In discussing the influence of steric features on mechanical properties, it is convenient to consider the
side chains and the main chain separately. The effects of flexible side chains differ completely from
those of stiff side chains. Long, flexible side chains reduce the glass transition temperature, while stiff
side chains increase it. Long, flexible side chains increase the free volume and ease the steric hindrance
from neighboring chains and as such facilitate the movement of the main chain. Figure 13.28 illustrates
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POLYMER SCIENCE AND TECHNOLOGY
Figure 13.28 Shear modulus (a) and damping (b) at 1 Hz as a function of temperature for poly(n-alkyl
methacrylate). (– – – –) Polymethyl methacrylate; (– – –) polyethyl methacrylate; (— — —) poly(n-propyl methacrylate); (· · · ·) poly(n-butyl methacrylate). (From Heijboer, J., Proc. Int. Conf. Physics Non-crystalline Solids,
North-Holland, Amsterdam, 1965, 231. With permission.)
the reduction in Tg and the increase in modulus in the glassy region with increase in length of the alkyl
group for poly(n-alkyl methacrylate).
Branched side chains, particularly if the branched point is located close to the main chain, increase
the glass transition temperature. This is illustrated in Figures 13.29 and 13.30 for a series of polyacrylates
and polyolefins with branched side chains, respectively.
Structural changes within or near the main chain, even if minor, can produce a drastic effect on the
mechanical properties of a polymer. Table 13.4 shows the increase in Tg and Tm (for the crystallizable
polymers) by the introduction of rings into the main chain.
In spite of the possible increase in chain flexibility due to the elongation of the diacid by two methylene
groups, polycyclamide has higher Tg and Tm than nylon 6,6 as a result of the substitution of stiff
cyclohexylene 1,4 for four methylene groups in nylon 6,6. Trogamid T does not crystallize. However,
the additional stiffening of the main chain due to the presence of methyl side groups leads to a further
increase in the Tg. In general, the introduction of rings into the main chain provides a better structural
mechanism for toughening polymers than chain stiffening by bulky side groups. For example, polycarbonate, polysulfone, and polyphenylene oxide all have high impact strength whereas poly(vinyl carbozole) with comparable Tg is brittle (Table 13.5).
H. EFFECT OF TEMPERATURE
The mechanical properties of polymers are generally more susceptible to temperature changes than those
of ceramics and metals. As discussed in Section 12.5.1, the modulus of a polymer decreases with
increasing temperature. However, the rate of decrease is not uniform; the drop in modulus is more
pronounced at temperatures associated with molecular transitions. As the temperature is increased to a
level that can induce some form of molecular motion, a relaxation process ensues and there is a drop
in modulus. At temperatures below the Tg, the polymer is rigid and glassy. In the glassy region, there
are usually one or more secondary transitions that are related either to movements of side chains or to
restricted movements of small parts of the main chain. For the secondary transitions, the drop in modulus
is about 10 to 50%, while primary transitions like Tg and Tm, which involve large-scale main chain
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MECHANICAL PROPERTIES OF POLYMERS
COO
25
Brittleness Temperature, °C
COO
H
CH3
H
COOCH2CH2
0
COOCH(CH2)n-4 CH3
-25
COO(CH2)n-3 CH3
-50
CH3
COOCH2CH(CH2)n-5 CH3
2
4
6
8
Length of side-chain (number of atoms, n)
10
Figure 13.29 Brittleness temperatures for polyacrylates as a function of the total length of the side chain. (From
Heijboer, J., Br. Polym. J., 1, 3, 1969. With permission.)
motions, the drop in modulus varies from one to three orders of magnitude depending on the type of
polymer. If no molecular relaxation process occurs over a particular temperature range, the modulus
decreases rather slowly with increasing temperature. In this case, the decrease in modulus is due to the
normal reduction in intermolecular forces that occurs with an increase in temperature.
The general effect of temperature on the stress-strain properties of a polymer is illustrated in
Figure 13.31. Below the Tg, the modulus is high, as discussed above; there is essentially no yield point
and consequently the polymer is brittle (i.e., has low elongation at break). The yield point appears at
temperatures close to the Tg. As we shall see in the next section, a high speed of testing requires higher
temperatures for the appearance of the yield point. In some polymers with pronounced secondary
transitions, the yield point appears in the neighborhood of this transition temperature rather than at Tg.
For example, in spite of a high Tg of 150°C, polycarbonate is remarkably tough at room temperature,
and this is associated with a secondary transition at about –100°C. In general, therefore, as the temperature
is increased, the modulus and yield strength decrease and the polymer becomes more ductile.
I. EFFECT OF STRAIN RATE
Polymers are very sensitive to the rate of testing. As the strain rate increases, polymers in general show
a decrease in ductility while the modulus and the yield or tensile strength increase. Figure 13.32 illustrates
this schematically. The sensitivity of polymers to strain rate depends on the type of polymer: for brittle
polymers the effect is relatively small, whereas for rigid, ductile polymers and elastomers, the effects
can be quite substantial if the strain rate covers several decades.
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POLYMER SCIENCE AND TECHNOLOGY
+ 100
(CH2)n-4
&
(CH2)n-4
(CH2)n-2 C(CH3)3
H
TS, °C
+ 50
(CH2)n-2 CH(CH3)2
0
(CH2)n-1 CH3
- 50
0
2
4
6
Length of side-chain (number of atoms, n)
8
Figure 13.30 Softening temperature of polyolefins with branched side chains. (From Heijboer, J., Br. Polym. J.,
1, 3, 1969. With permission.)
Table 13.4 Effects of the Introduction of Rings into the Main Chain of Some Polyamides
Polymer
Structure
O
Nylon 6.6
(Zytel, Du Pont de Nemours)
C
(CH2)4
O
H
C
N
Tg °C
Tm °C
75
260
125
300
145
none
H
(CH2)6
N
n
O
Polycyclamide
(Q.2 Tennessee Eastman Co.)
C
(CH2)6
O
H
C
N
H
CH2
CH2
N
n
H
Trogamid T
(Dynamit Nobel A.G.)
N
CH3 CH3
CH2
C
CH3
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CH
(CH2)2
H
O
O
N
C
C
n
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Table 13.5 Polymer Stiffening Due to the Introduction of Rings into the Main Chain
Polymer
Tg °C
Structure
CH3
Polycarbonate
O
C
O
O
C
150
CH3
CH3
Polysulfone
O
O
C
O
S
O
CH3
Poly(phenylene oxide)
CH3
CH3
CH3
CH3
O
O
O
O
CH3
CH3
CH3
CH3
CH2
Poly(vinyl carbazole)
190
220
CH
N
208
Polymers show a similar response to temperature and strain rate (time), as might be expected from
the time–temperature superposition principle (compare Figures 13.31 and 13.32). Specifically, the effect
of decreasing temperature is equivalent to that of increasing the strain rate. As has become evident from
our previous discussions, low temperature restricts molecular movement of polymers, and consequently
they become rigid and brittle. Materials deform to relieve imposed stress. High strain rates preclude
such deformation and therefore result in brittle failure.
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POLYMER SCIENCE AND TECHNOLOGY
Figure 13.31 The stress–strain behavior of cellulose acetate at different temperatures. (From Carswell, T.S.
and Nasor, H.K., Mod. Plast., 21(6), 121, 1944. With permission.)
Figure 13.32
Copyright 2000 by CRC Press LLC
Schematic illustration of the effect of strain rate on polymers.
MECHANICAL PROPERTIES OF POLYMERS
375
Example 13.7: Explain the observed differences in the tan δ of polystyrene tested at the two different
frequencies shown in Figure E13.7.
Figure E.13.7 Schematic representation of the variation of tan δ with temperature for atactic polystyrene tested
at 0.1 Hz (——) and 50 Hz (– – –).
Solution: The mechanical properties of polymers depend on time and temperature. The time dependence
is usually expressed as a frequency dependence, which to a first approximation is related to time by 2π
υ = 1/t where υ is the frequency. The combined dependence of molecular processes of viscoelastic materials
on frequency and temperature can be described by an activation energy Ea. Ea is about 100 kcal/mol and
10 kcal/mol for primary and secondary transitions, respectively. This implies that the relaxation processes
associated with the molecular motions shift to higher temperatures at higher frequencies; however, the
secondary transition shifts more than the primary transition. Therefore, if tests are conducted at high
frequencies, the resolution between the energy absorption peaks for primary and secondary transitions
that are close to each other is poor. Thus, in this case, the β and α peaks, which are relatively distinct
at 0.1 Hz, merge at 50 Hz, and there is a shift in the peak to higher temperatures.
J. EFFECT OF PRESSURE
The imposition of hydrostatic pressure on a polymer has a tremendous effect on its mechanical properties,
as demonstrated by the stress–strain behavior of polypropylene in Figure 13.33. The modulus and yield
stress increase with increasing pressure. This behavior is general for all polymers. However, the effect
of pressure on the tensile strength and elongation at break depends on the polymer. The tensile strength
tends to increase for ductile polymers, but decrease for some brittle polymers. The elongation at break
increases for some ductile polymers, but decreases for most brittle polymers and polymers such as PE,
PTFE, and PP, which exhibit cold drawing at normal pressures. In some brittle polymers like PS, brittleductile transition is induced beyond a certain critical pressure.
The increase in modulus and yield stress with increase in pressure is to be expected on the basis of
the available free volume. An increase in pressure decreases the free volume (i.e., increases the packing
density) and as such enhances the resistance to deformation (modulus) or delays the onset of chain
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POLYMER SCIENCE AND TECHNOLOGY
Figure 13.33 The stress–strain behavior of polypropylene at different pressures. (From Nielsen, L.E., Mechanical Properties of Polymers and Composites, Vol. 2, Marcel Dekker, New York, 1974. With permission.)
slippage or plastic deformation (yielding). The embrittlement of polymers capable of cold drawing with
increase in pressure can be rationalized on the basis of the reduction in chain mobility associated with
the decrease in free volume. Also, with an increase in pressure, the occurrence of relaxation processes
responsible for the cold drawing phenomenon is delayed to higher temperatures. In other words, subjecting a polymer to increasing pressure at a given temperature has an effect on its mechanical behavior
equivalent to reducing its temperature at a given pressure.
VII.
POLYMER FRACTURE BEHAVIOR
Structural members can fail to perform their intended functions in three general ways: (1) excessive
elastic deformation, (2) excessive plastic deformation, and (3) fracture. Modulus is a measure of the
resistance to elastic deformation. We discussed the effects of various factors on modulus in the previous
section. The stress levels encountered in most end-use situations preclude excessive plastic deformation.
Although polymers are noted generally for the ductility, they are susceptible to brittle failure under
appropriate conditions. Brittle fracture is usually catastrophic and involves very low strains. The major
conditions responsible for brittle failure include low temperature, high rates of loading such as during
shock or impact loading, and alternating loads. A polymer can rupture at loads much lower than would
be dictated by its yield or tensile stress if subjected to alternating loads or to static loads for long duration.
Therefore, to utilize a polymer properly, careful consideration must be given to possible brittle failure
that can arise from environmental conditions and/or stress levels imposed on the polymer in use. We
now discuss very briefly the fracture behavior in polymers.
A. BRITTLE FRACTURE
The theoretical fracture strength of a material can be deduced from the cohesive forces between the
component atoms in the plane under consideration from a simple energy balance between the work to
fracture and the energy require to create two new surfaces. It can be shown that the theoretical cohesive
strength is given by
σc =
Copyright 2000 by CRC Press LLC
Eγs
a0
(13.31)
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MECHANICAL PROPERTIES OF POLYMERS
where σC
E
γS
a0
= theoretical cohesive strength
= Young’s modulus
= surface energy per unit area
= equilibrium interatomic spacing of atoms in the unstrained state
Engineering materials, including polymers, generally have low fracture strengths relative to their
theoretical capacity. The lower-than-ideal fracture strengths of engineering materials are generally attributed to the presence of flaws such as cracks, scratches, and notches inherent in these materials.
The first explanation of the discrepancy between observed fracture strength of crystals and the
theoretical cohesive strength was proposed by Griffith.12 He utilized an earlier analysis by Inglis,13 who
showed that the applied stress, σo , was magnified at the ends of the major axis of an elliptical hole in
an infinitely wide plate (Figure 13.34) according to the following relation:
(
σt = σ0 1 − 2 a ρ
)
(13.32)
where ρ is the radius of curvature of the tip of the hole, 2a is the length of the major axis of the hole,
and σt is the stress and the end of the major axis.
Griffith proposed that a brittle material contains a population of cracks and that the amplification of
the local stresses at the crack tip was such that the theoretical cohesive strength was reached at nominal
stress levels much below the theoretical value. The high stress concentrations lead to an extension of
one of the cracks. The creation of two new surfaces results in a concomitant increase in surface energy,
which is supplied by a decrease in the elastic strain energy. Griffith established the following criterion
for the propagation of a crack: “A crack will propagate when the decrease in the elastic strain energy
Figure 13.34 Model of an elliptical crack of length 2a subjected to a uniform stress σ0 in an infinite plate.
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POLYMER SCIENCE AND TECHNOLOGY
is at least equal to the energy required to create the new surface.”12 He showed that the stress required
for crack propagation is given by the relation:
2E γs
πa
σf =
σf = −
(
for plane stress
2E γ s
)
for plane strain
1 − ν 2 (πa )
(13.33)
(13.34)
where E is Young’s modulus, γs is the surface energy per unit area, ν is Poisson’s ratio and a is half
crack length.
The Griffith relation was derived for an ideally brittle material and the theory satisfactorily predicts
the fracture strength of a material like glass, where the work to fracture is essentially equal to the increase
in surface energy. On the other hand, the theory is inadequate for describing the fracture behavior of
materials like polymers and metals, which are capable of extensive plastic deformation (i.e., materials
whose fracture energies are several orders of magnitude greater than surface energy). A substantial
amount of local plastic deformation invariably occurs at the crack tip.
To obviate this inadequacy, Griffith’s equation had to be modified to include the energy expended in
plastic deformation in the fracture process. Accordingly, Irwin14 defined a parameter, G, the strain-energyrelease rate or crack extension force, which he showed to be related to the applied stress and crack
length by the equation:
EG
πa
σ=
(13.35)
At the point of instability, the elastic energy release rate G reaches a critical value, GC, whereupon a
previously stationary crack propagates abruptly, resulting in fracture.
A different approach from the strain-energy-release rate in the study of the fraction process is the
analysis of the stress distribution around the crack tip. This gives the stress intensity factor or fracture
toughness K, which for a sharp crack in an infinitely wide and elastic plate is given by
K = σ πa
(13.36)
The stress intensity factor K is a useful and convenient parameter for describing the stress distribution
around a crack. The difference between one cracked component and another lies in the magnitude of K.
If two flaws of different geometries have the same value of K, then the stress fields around each of the
flaws are identical. Since K is a function of applied load and crack size, it increases with load. When
the intensity of the local tensile stresses at the crack tip attains a critical value, KC, failure occurs. This
critical value, KC, defines the fracture toughness and is constant for a particular material, since cracking
always occurs at a given value of local stress intensity regardless of the structure in which the material
is used. KC is a function of temperature, strain rate, and the state of stress, varying between the extremes
of plane stress and plane strain. In addition, KC also depends on the failure mode. The stress field
surrounding a crack tip can be divided into three major modes of loading depending on the crack
displacement, as shown in Figure 13.35. Mode I is the predominant mode of failure in real situations
involving cracked components. This is the mode of failure that has received the most extensive attention.
For mode I, G and K are related according to Equations 13.37 and 13.38.
G Ic =
G IC =
Copyright 2000 by CRC Press LLC
K 2IC
E
(plane stress)
2
K IC
1 − ν2
E
(
) (plane strain)
(13.37)
(13.38)
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MECHANICAL PROPERTIES OF POLYMERS
Figure 13.35 Basic modes of failure of structural materials: (I) opening or tensile mode; (II) sliding or in-plane
shear mode; (III) tearing or antiplane shear mode.
Figure 13.36 (a) Schematic of a specimen loading in mode I in Figure 13.35; (b) schematic of a corresponding
load-displacement diagram.
B. LINEAR ELASTIC FRACTURE MECHANICS (LEFM)
Our discussion thus far has focused in a rather superficial way on the general evolution of the important
area of fracture mechanics. The basic objective of fracture mechanics is to provide a useful parameter
that is characteristic of the given material and independent of test specimen geometry. We will now
consider how such a parameter, such as GIc, is derived for polymers. In doing so we confine our discussion
to the concepts of linear elastic fracture mechanics (LEFM). As the name suggests, LEFM applies to
materials that exhibit Hookean behavior.
To determine GIc, consider a precracked specimen (Figure 13.36a). In order to obtain the energy lost
to a growing crack, we should examine the energy stored in the system before and after crack extension.
This can be done by using the load (P)–displacement (ε) diagram (Figure 13.36b) and calculating the
total energy before and after a finite amount of crack motion. This difference becomes, in the limit, the
value of GIc .
The total energy U(a1) available to the specimen for an initial crack length, a, and a critical load, Pc,
is given by
U (a 1 ) =
Pc ε (a1 )
2
(13.39)
where ε is the displacement. The reciprocal of the slope of P–ε line is defined as the compliance of the
specimen
C(a1 ) = (a1 ) Pc
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(13.40)
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POLYMER SCIENCE AND TECHNOLOGY
Substituting (13.40) into (13.39) we obtain
U (a 1 ) =
Pc2 C(a1 )
2
(13.41)
Assuming the crack extends infinitesimally at constant load, the expression for the energy lost to the
growing crack per unit area for a width, b, is
G IC =
dU 1 dU Pc2 ∂ C
=
=
dA b da 2 b ∂ a
(13.42)
Equation 13.42 is general and can be used to determine the cohesive fracture behavior of polymers. To
do this, a large number of specimen geometries have been devised. However, we illustrate the use of
Equation 13.42 in the assessment of the strength of adhesive joints using tapered double-cantilever beam
(TDCB) specimen geometry. Most adhesive materials are thermosets, which are usually highly cross-linked
and brittle. Thermosets are therefore ideally suited for the application of the principles of linear elastic
fracture mechanics. As indicated earlier, structural members can fail to perform their intended functions
by excessive elastic deformation, excessive plastic deformation, or fracture. Adhesive joints constitute a
minute fraction of the total volume of a structure. Consequently, even large elastic or inelastic deformation
within the bond line is not hazardous since their contribution to the overall deformation of the structure
would be insignificant. However, rigid structural adhesives are particularly susceptible to brittle failures;
hence, prevention of brittle fracture is the critical problem in adhesive-bonded structures.
Fracturing in brittle materials, we have seen from theories discussed above, is associated with the
occurrence of preexisting flaws. These initial flaws, introduced in manufacture or service, may be dust
particles, bubbles, and nonbonded areas in the case of adhesive joints. Fractures usually occur by a
progressive extension of the largest of these flaws. Furthermore, structural adhesives are inherently brittle
when tested uniaxially, a characteristic that is accentuated when they are used in thin layers where
deformation is further restricted because of the multiaxial stress state imposed by the proximity of high
modulus adherends. The combination of fracturing with very little permanent deformation and crack
growth from preexisting flaws suggests that fracturing of adhesive joints can be described by the
techniques of fracture mechanics. Using the concepts of linear elastic fracture mechanics and with a
knowledge of the largest flaw contained in these materials, one can establish minimum toughness
standards for structural adhesive joints. Mostovoy et al.15,16 and Ripling et al.17–19 have developed and
applied the tapered double-cantilever beam (TDCB) to study strength and durability characteristics of
adhesives and adhesive joints with metal adherends. Using beam theory from strength of materials, they
established the following relation between specimen compliance and crack length.
∂C
8
=
∂a Eb
where E
ν
b
h
3a 2 1
h3 + h
(13.43)
= Young’s modulus
= Poisson’s ratio (assumed to be 1/3)
= specimen width
= beam height at the distance a from the point of loading
From Equations 13.42 and 13.43,
G IC =
4 Pc2 3a 2 1
+
EB2 h 3 h
(13.44)
From Equation 13.44, it can be seen that as the crack gets longer, i.e., as a increases, Pc decreases to
maintain a constant value of GIc . Obviously, the calculation of GIc requires monitoring both Pc and a for
each calculation of GIc . Testing can be simplified, however, if the specimen is contoured so that the
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381
MECHANICAL PROPERTIES OF POLYMERS
Figure 13.37 Schematic representation of TDCB specimens for testing of A) bulk materials and B) adhesives.
compliance changes linearly with crack length. If dC/da is a constant, the relation between GIc and Pc
is independent of a and, hence, only Pc needs to be followed for the evaluation of GIc . To develop a
linear compliance specimen, its height is varied so that the quantity (3a2/h3 + 1/h) in Equation 13.44 is
constant. Hence
3a 2 1
+ = m (a constant)
h3 h
(13.45)
so that
G IC =
4 Pc2
⋅m
Eb 2
(13.46)
This type of contoured specimen is known as the tapered-double-cantilever beam (TDCB) specimen.
There are, of course, an infinite number of m values that can be chosen to satisfy Equation 13.46.
The determination of m is governed by a basic assumption that the entire energy supplied to the specimen
is concentrated on the crack line for crack extension. Thus the choice of m is such that the bending
stresses on the adherend are minimized. This, in turn, depends on the modulus of the adherends relative
to the adhesive. This necessitates an empirical determination of the appropriate m value through a
procedure known as compliance calibration. Figure 13.37 is a schematic of the specimen geometries of
TDCB for the determination of GIc for bulk adhesives and adhesive joints.
The shape of the load–displacement curves using this specimen geometry can reveal interesting
information about the intrinsic nature of the adhesive. Generally, in evaluating GIc, a continuously
increasing load is applied to the specimen and a P–ε diagram is obtained with the extensometer mounted
directly on the sample. Two types of P–ε diagrams are usually obtained (Figure 13.38). Curves of type
A are typical of rate-insensitive materials, while type B curves exemplify rate-sensitive ones. For type
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POLYMER SCIENCE AND TECHNOLOGY
B
Figure 13.38 Load–displacement (P–ε) diagrams typical of (A) stable (“flat”) propagation; (B) unstable
(“peaked”) propagation.
B curves, two load values are exhibited: Pc , the crack initiation load at the point of instability used in
the calculation of GIc ; and Pa , the crack arrest load used for estimating GIa . For rate-insensitive materials,
·
GIc and GIa are identical, and the crack growth a is dictated by the cross-head speed, ε.
On a given test machine and at a given cross-head speed, the difference between initiation and arrest
fracture energies, GIc – GIa indicates the energy released during crack propagation; it is a measure of
the brittleness or resistance to catastrophic failure of the adhesive system. Figure 13.39 shows different
adhesive joint failure modes for TDCB specimens. We notice that the systems A and B have about the
same crack initiation loads. However, the two systems are different in their resistance to crack propagation. A is a hard and very brittle adhesive. Unlike A, B has an internal mechanism for arresting crack
propagation; at the onset of instability the material undergoes plastic deformation, blunts the crack, and
consequently prevents catastrophic failure. B is therefore not only hard but also tough. Using these
arguments, the adhesive system C is hard and strong while D is soft and weak. D is characteristic of
systems with a spongy adhesive layer.
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MECHANICAL PROPERTIES OF POLYMERS
Figure 13.39 Possible adhesive joint failure modes for TDCB specimens tests.
Example 13.8: Urea–formaldehyde (UF) adhesive used in bonded wood products was modified by
copolymerizing 10 mol% urea derivative of dodecanediamine (DDDU). The fracture energies of wood
joints made with the unmodified and DDDU-modified adhesives were found to be 130 and 281 J/m3,
respectively. Explain the enhanced fracture energy of the wood joint bonded with the modified adhesive.
Solution:
H 2 N − CO − NH − (CH 2 )12 − NH − CO − NH 2
urea derivative of dodecanediamine (DDDU)
Cured urea–formaldehyde adhesive is characterized by the presence of methylene bridges between
strongly hydrogen-bonded urea linkages. Consequently, cured UF adhesives are inherently stiff and
brittle. Incorporation of DDDU with its 12 methylene groups into the resin structure results in cured UF
adhesive with a more flexible network. The increased flexibility decreases internal stress and the associated flaws, and hence the fracture energy increases.
VIII.
PROBLEMS
13.1. Calculate the resilience of polycarbonate with yield strength 62.05 × 106 N/m2 and elastic modulus
24.13 × 108 N/m2.
13.2. A polymer, T, when tested in uniaxial tension has its cross-sectional area reduced from 2.00 cm2 to
1.0 cm2. The cross-sectional area of a second polymer, S, under the same test changed from 20 cm2 to
18 cm2. Which of these two polymers would you choose for making children’s toys, assuming other
qualities are comparable for both polymers?
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POLYMER SCIENCE AND TECHNOLOGY
13.3. Explain the observed trend in the mechanical properties of the polymers shown in the following table.
Polymer
Low-density polyethylene
High-density polyethylene
Polytetrafluoroethylene
Polypropylene
Polystyrene
Poly(vinyl chloride)
Nylon 6,6
Polycarbonate
Elastic modulus
(MN/m2)
Tensile Strength
(MN/m2)
Elongation of Break
%
138–276
414–1034
414
1034–1551
2758–3447
2068–4436
1241–2760
1243
10.3–17.2
17.2–39.9
13.8–27.6
24.1–37.9
37.9–55.2
41.4–75.8
62.0–82.7
55.2–68.9
400–700
100–600
100–350
200–600
1–2.5
5–60
60–300
60–120
13.4. Figure Q13.4 shows the stress–strain curve for a polymer material under uniaxial loading. The material
deformed uniformly until N, where necking ensued. If the material obeys the relation σ = Kεn up to the
point N, calculate the strength, σN, of the material at the onset of necking. K = 106 N/m2, ε = 0.5.
Figure Q13.4 Stress–strain curve for a polymer.
13.5. Calculate the maximum load a polymer sample in uniaxial tension can sustain before yielding when
the maximum resolved shear stress (τmax) is 106 N/m2. The cross-sectional area of the sample is 10–4 m2.
13.6. Suppose that yielding occurs in a polyethylene crystal when the critical resolved shear stress, τmax, ≈
27.58 × 106 N/m2 is produced on the {110} type slip plane along a <111> type direction. If the tensile
axis coincides with the <110> direction, what maximum axial stress must be applied to cause yielding
on a {110} plane in a <111> direction?
13.7. Explain the following observations:
a. Although the refractive indices of polystyrene and polybutadiene differ considerably, materials
derived from styrene–butadiene–styrene copolymers are transparent and have high tensile strength.
b. When natural rubber with chains of poly(methylmethacrylate) grafted to it is isolated from ethylacetate, it is hard, stiff, and nontacky. On the other hand, when the same polymer is isolated from
hexane it is limp, flabby, and tacky.
c. Block copolymers of styrene and vinyl alcohol are soluble in benzene, water, and acetone.
d. Both poly(hexamethylene sebacamide) (nylon 6,10) and poly(hexamethylene adipamide) (nylon 6,6)
are fiber-forming polymers. A copolymer consisting of hexamethylene terephthalamide and hexamethylene adipamide retains the qualities of nylon 6,6 fiber, whereas a copolymer consisting of hexamethylene sebacamide and 10 mol% hexamethylene terephthalamide is elastic and rubbery with drastic
reductions in stiffness and hardness.
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MECHANICAL PROPERTIES OF POLYMERS
13.8. Two polypropylene materials have densities 0.905 and 0.87 g/cm3. Sketch the modulus–temperature
curves for these materials on the same graph.
13.9. Explain the difference in the glass transition temperatures between the following pairs of polymers.
a. Poly(vinyl methyl ether) and poly(vinyl formal)
Tg = 13°C
Tg = 105°C
b. Poly(t-butyl methacrylate) and poly(ethyl methacrylate)
Tg = 135°C
Tg = 100°C
13.10. How would you expect the stiffness of the series of poly(p-alkylstyrene) shown below to vary with n?
Explain your answer very briefly.
CH2
CH
(CH2)n
CH3
(Str. 11)
13.11. Two materials formed by reaction injection molding have the following tensile and fracture properties:26
Material
E(MPa)
σu(MPa)
εu(%)
Gc(kJ/m2)
S
H
266
532
16.8
15.3
222
17
7.12
0.06
Estimate the intrinsic flaw size in these materials assuming they were tested under (a) plane stress
and (b) plane strain. Assume ν = 0.3. Comment on your result. Which of these two materials will be
more suitable for use in an application where the ability to withstand extensive abuse is a requirement?
13.12. Figure Q13.12 shows the mechanical properties of two urea–formaldehyde (UF) resins cured with
NH4Cl: (bottom) variation of the shear strength of bonded wood joints with cyclic wet–dry treatments
of joints; (middle) development of internal stress with duration of resin cure at room temperature;
(top) dynamic mechanical properties of resins. Discuss the interrelationships between the observed
mechanical properties.
13.13. Figure Q13.13 shows the variation of fracture initiation (GIc) energies with cure of phenol–formaldehyde adhesive cured at 85°C. It is known that during cure of phenolic resins, dimethylene ether
linkages are formed initially from the condensation of methylol groups. These ether linkages decompose subsequently according to the scheme shown. How do these reactions provide a possible
explanation for the trend in GIc? A and B represent two different states of cure of the phenolic resin
with the same fracture energy. What is the major difference in the fracture characteristics of materials
in these states?
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POLYMER SCIENCE AND TECHNOLOGY
Figure Q13.12
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MECHANICAL PROPERTIES OF POLYMERS
387
Figure Q13.13 Variation of fracture initiation and arrest energies with cure time for phenolic resin cured at 85° ( ).
REFERENCES
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Findley, W.N., Mod. Plast., 19(8), 71, 1942.
McLoughlin, J.R. and Tobolsky, A.V., J. Colloid Sci., 7, 555, 1952.
Fried, J.R., Plast. Eng., 38(7), 27, 1982.
Williams, D.J., Polymer Science and Technology, Prentice-Hall, Englewood Cliffs, NJ, 1971.
Winding, C.C. and Hiatt, G.D., Polymer Materials, McGraw-Hill, New York, 1961.
Kaufman, H.S. and Falcetta, J.J., (Eds.), Introduction to Polymer Science and Technology, John Wiley & Sons, New
York, 1977.
Nielsen, L.E., Mechanical Properties of Polymers and Composites, Vol. 2, Marcel Dekker, New York, 1974.
Barrett, C.R., Nix, W.D., and Tetelman, A.S., The Principles of Engineering Materials, Prentice-Hall, Englewood
Cliffs, NJ, 1973.
Heijboer, J., Br. Polym. J., 1, 3, 1969.
Heiboer, J., Proc. Int. Conf. Physics Non-crystalline Solids, North-Holland, Amsterdam, 1965, 231.
Carswell, T.S. and Nasor, H.K., Mod. Plast., 21(6), 121, 1944.
Griffith, A.A., Philos. Trans. R. Soc. London, A221, 163, 1920.
Inglis, C.E., Proc. Institute of Naval Architects, 55, 219, 1913.
Irwin, G.R. and Wells, A.A., Metall. Rev., 10(58), 223, 1965.
Mostovoy, S., Crosley, P.B., and Ripling, E.J., J. Mater., 2(3), 661, 1967.
Mostovoy, S. and Ripling, E.J., J. Appl. Polym. Sci., 15, 641, 1971.
Ripling, E.J., Mostovoy, S., and Patrick, R. L., Adhesion ASTM STP 360, 5, 1963.
Ripling, E.J., Mostovoy, S., and Patrick, R.L., Mater. Res. Stand., 64(3), 129, 1964.
Ripling, E.J., Corten, H.T., and Mostovoy, S., J. Adhes., 3, 107, 1971.
Ryan, A.J., Bergstrom, T.D., Willkom, W.R., and Macosko, C.W., J. Appl. Polym. Sci., 42, 1023, 1991.
Copyright 2000 by CRC Press LLC
Chapter 14
Polymer Viscoelasticity
I.
INTRODUCTION
Traditional engineering practice deals with the elastic solid and the viscous liquid as separate classes of
materials. Engineers have been largely successful in the use of materials like motor oil, reinforced
concrete, or steel in various applications based on design equations arising from this type of material
classification. However, it has become increasingly obvious that elastic and viscous material responses
to imposed stresses represent the two extremes of a broad spectrum of material behavior. The behavior
of polymeric materials falls between these two extremes. As we said in Chapter 13, polymers exhibit
viscoelastic behavior. The mechanical properties of solid polymers show marked sensitivity to time
compared with traditional materials like metals and ceramics. Several examples illustrate this point.
(1) The stress–strain properties of polymers are extremely rate dependent. For traditional materials, the
stress–strain behavior is essentially independent of strain rate. (2) Under a constant load, the deformation
of polymeric material increases with time (creep). (3) When a polymer is subjected to a constant
deformation, the stress required to maintain this deformation decreases with increasing time (stress
relaxation). (4) The strain resulting from a polymer subjected to a sinusoidal stress has an in-phase
component and an out-of-phase component. The phase lag (angle) between the stress and strain is a
measure of the internal friction, which in principle is the mechanical strain energy that is convertible to
heat. Traditional materials, for example, metals close to their melting points, exhibit similar behavior.
However, at normal temperatures, creep and stress relaxation phenomena in metals are insignificant and
are usually neglected in design calculations. In choosing a polymer for a particular end-use situation,
particularly structural applications, its time-dependent behavior must be taken into consideration if the
polymer is to perform successfully.
Our discussion of the viscoelastic properties of polymers is restricted to the linear viscoelastic behavior
of solid polymers. The term linear refers to the mechanical response in which the ratio of the overall
stress to strain is a function of time only and is independent of the magnitudes of the stress or strain
(i.e., independent of stress or strain history). At the onset we concede that linear viscoelastic behavior
is observed with polymers only under limited conditions involving homogeneous, isotropic, amorphous
samples under small strains and at temperatures close to or above the Tg. In addition, test conditions
must preclude those that can result in specimen rupture. Nevertheless, the theory of linear viscoelasticity,
in spite of its limited use in predicting service performance of polymeric articles, provides a useful
reference point for many applications.
To aid our visualization of viscoelastic response we introduce models that represent extremes of the
material response spectrum. This is followed by the treatment of mechanical models that simulate
viscoelastic response. These concepts are developed further by discussion of the superposition principles.
II.
SIMPLE RHEOLOGICAL RESPONSES
A. THE IDEAL ELASTIC RESPONSE
The ideally elastic material exhibits no time effects and negligible inertial effects. The material responds
instantaneously to applied stress. When this stress is removed, the sample recovers its original dimensions
completely and instantaneously. In addition, the induced strain, ε, is always proportional to the applied
stress and is independent of the rate at which the body is deformed (Hookean behavior). Figure 14.1
shows the response of an ideally elastic material.
The ideal elastic response is typified by the stress–strain behavior of a spring. A spring has a constant
modulus that is independent of the strain rate or the speed of testing: stress is a function of strain only.
For the pure Hookean spring the inertial effects are neglected. For the ideal elastic material, the
mechanical response is described by Hooke’s law:
0-8493-????-?/97/$0.00+$.50
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Figure 14.1 Ideal elastic response.
σ = Eε
(14.1)
where σ is the applied stress, ε is the strain, and E is Young’s modulus.
B. PURE VISCOUS FLOW
Fluids have no elastic character; they cannot support a strain. The dominant characteristic of fluids is
their viscosity, which is equivalent to elasticity in solids. According to Newton’s law, the response of a
fluid to a shearing stress τ is viscous flow, given by
τ=η
dγ
dt
(14.2)
where η is viscosity and dγ/dt is strain rate. Thus in contrast to the ideal elastic response, strain is a
linear function of time at an applied external stress. On the release of the applied stress, a permanent
set results. Pure viscous flow is exemplified by the behavior of a dashpot, which is essentially a piston
moving in a cylinder of Newtonian fluid (Figure 14.2). A dashpot has no modulus, but the resistance to
motion is proportional to the speed of testing (strain rate).
However, no real material shows either ideal elastic behavior or pure viscous flow. Some materials,
for example, steel, obey Hooke’s law over a wide range of stress and strain, but no material responds
without inertial effects. Similarly, the behavior of some fluids, like water, approximate Newtonian
response. Typical deviations from linear elastic response are shown by rubber elasticity and viscoelasticity.
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POLYMER VISCOELASTICITY
Figure 14.2 Pure viscous behavior.
C. RUBBERLIKE ELASTICITY
The response of rubbery materials to mechanical stress is a slight deviation from ideal elastic behavior.
They show non-Hookean elastic behavior. This means that although rubbers are elastic, their elasticity
is such that stress and strain are not necessarily proportional (Figure 14.3).
III.
VISCOELASTICITY
Viscoelastic material such as polymers combine the characteristics of both elastic and viscous materials.
They often exhibit elements of both Hookean elastic solid and pure viscous flow depending on the experimental time scale. Application of stresses of relatively long duration may cause some flow and irrecoverable
(permanent) deformation, while a rapid shearing will induce elastic response in some polymeric fluids. Other
examples of viscoelastic response include creep and stress relaxation, as described previously.
Figure 14.3 Rubber elasticity.
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Figure 14.4 The Maxwell element.
It is helpful to introduce mechanical elements as models of viscoelastic response, but neither the
spring nor the dashpot alone accurately describes viscoelastic behavior. Some combination of both
elements is more appropriate and even then validity is restricted to qualitative descriptions; they provide
valuable visual aids. In most polymers, mechanical elements do not provide responses beyond strains
greater than about 1% and strain rates greater than 0.1 s–1.
IV.
MECHANICAL MODELS FOR LINEAR VISCOELASTIC RESPONSE
A. MAXWELL MODEL
To overcome the poor description of real polymeric materials by either the spring or the dashpot, Maxwell
suggested a simple series combination of both elements. This model, referred to as the Maxwell element,
is shown in Figure 14.4. In the Maxwell model, E, the instantaneous tensile modulus, characterizes the
response of the spring while the viscosity, η, defines viscous response. In the following description we
make no distinction between the types of stress. Thus, we use the symbol E even in cases where we are
actually referring to shearing stress for which we have previously used the symbol G. This, of course,
does not detract from the validity of the arguments.
In the Maxwell element, both the spring and the dashpot support the same stress. Therefore,
σ = σa = σd
(14.3)
where σs and σd are stresses on the spring and dashpot, respectively. However, the overall strain and
strain rates are the sum of the elemental strain and strain rates, respectively. That is,
ε T = εs + ε d
(14.4)
ε«T = ε«6 + ε«d
(14.5)
σ«
ε«s = and ε«d = σ η
E
(14.6)
or
But
where ε· T is the total strain rate, while ε· s and ε· d are the strain rates of the spring and dashpot, respectively.
The rheological equation of the Maxwell element on substitution of Equation 14.6 in 14.5 becomes
1
1
ε«T = σ« + σ
E
η
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(14.7)
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POLYMER VISCOELASTICITY
As Equation 14.7 shows, the Maxwell element is merely a linear combination of the behavior of an
ideally elastic material and pure viscous flow. Now let us examine the response of the Maxwell element
to two typical experiments used to monitor the viscoelastic behavior of polymer.
1. Creep Experiment
In creep, the sample is subjected to an instantaneous constant stress, σo , and the strain is monitored as
a function of time. Since the stress is constant, dσ/dt is zero and therefore, Equation 14.7 becomes
1
ε«T = σ 0
η
(14.8)
Solving the equation and noting that the initial strain is σo /E, the equation for the Maxwell element for
creep can be written as
σ0 σ0
+
t
E
η
(14.9)
1 t
ε (t ) = σ 0 +
E η
(14.10)
ε (t ) =
On removal of the applied stress, the material experiences creep recovery. Figure 14.5 shows the creep
and the creep recovery curves of the Maxwell element. It shows that the instantaneous application of a
constant stress, σo , is initially followed by an instantaneous deformation due to the response of the spring
by an amount σo /E. With the sustained application of this stress, the dashpot flows to relieve the stress.
The dashpot deforms linearly with time as long as the stress is maintained. On the removal of the applied
stress, the spring contracts instantaneously by an amount equal to its extension. However, the deformation
due to the viscous flow of the dashpot is retained as permanent set. Thus the Maxwell element predicts
that in a creep/creep recovery experiment, the response includes elastic strain and strain recovery, creep
and permanent set. While the predicted response is indeed observed in real materials, the demarcations
are nevertheless not as sharp.
2. Stress Relaxation Experiment
In a stress relaxation experiment, an instantaneous strain is applied to the sample. The stress required
to maintain this strain is measured as a function of time. When the Maxwell element is subjected to an
Figure 14.5 Creep and creep recovery behavior of the Maxwell element.
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Figure 14.6 Relaxation time for the Maxwell element.
instantaneous strain, only the spring can respond initially. The dashpot will relax gradually, and consequently, the stress decreases with increasing time.
The rheological equation for the Maxwell element from Equation 14.7 is
1
1
ε«T = σ« + σ
E
η
(14.7)
Since the strain is constant, εT is zero, thus Equation 14.7 is reduced to
1 « 1
σ+ σ=0
E
η
(14.11)
The solution to this first-order differential equation with the boundary condition that σ = Eε0 at t = 0 is
E
σ = σ 0 exp − t
η
(14.12)
We define the quantity τ as the relaxation or response time and it is given as the ratio η/E. Equation 14.12
thus becomes
σ = σ 0 exp (− t τ)
(14.13)
The relaxation time from Equation 14.13 is the time required for the stress to decay to 1/e or 37% of
its initial value (Figure 14.6). If we divide the stress by the constant strain, ε0, Equation 14.13 becomes
σ (t ) σ 0 − t τ
=
e
ε0
ε0
or
E r (t ) = Ee − t τ
(14.14)
Er is the relaxation modulus. For the Maxwell element in a stress relaxation experiment, all the initial
deformation takes place in the spring. The dashpot subsequently starts to relax and allows the spring to
contract. For times considerably shorter than the relaxation time, the Maxwell element behaves essentially
like a spring; while for times much longer than the relaxation time, the element behaves like a dashpot.
For times comparable to the relaxation time, the response involves the combined effect of the spring
and the dashpot.
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POLYMER VISCOELASTICITY
Example 14.1: A polystyrene sample of 0.02/m2 cross-sectional area is subjected to a creep load of
105 N. The load is removed after 30 s. Assuming that the Maxwell element accurately describes the
behavior of polystyrene and that viscosity is 5 × 1010 P, while Young’s modulus is 5 × 105 psi, calculate:
a. The compliance
b. The deformation recovered on the removal of the dead load
c. The permanent
Solution: a. The creep equation for the Maxwell element is
1 t
ε (t ) = σ 0 +
E η
or
ε (t ) 1 t
= + or compliance.
σ0
E η
(
E = 5 × 10 5 psi = 5 × 10 5 psi × 6.894 × 103 N m 2 psi
)
= 3.45 × 10 9 N m 2
η = 5 × 1010 P = 5 × 10 9
J=
N ⋅s
m2
1
30
+
m 2 N ⋅ = 6.29 × 10 −9 m 2 N ⋅
3.45 × 10 9 5 × 10 9
b. The deformation recovered on the removal of load is due to the spring εs .
εs =
σ0
E
σ0 =
P0
10 5
=
N m 2 = 5 × 10 6 N m 2
A 0.02
εs =
5 × 10 6
= 1.45 × 10 −3
3.45 × 10 9
c. The permanent set is due to the viscous flow εd.
εd =
=
σ0
t
η
( ) × 30 (s)
(Ns m )
5 × 10 6 N m 2
5 × 10
9
2
= 0.03
3. Dynamic Experiment
Let us consider the response of a Maxwell element subjected to a sinusoidal stress. The corresponding
strain will be sinusoidal but out of phase with the stress by an angle δ, as discussed in Chapter 13. Thus,
σ = σ 0 sin ⋅ ω t
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(14.15)
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Now the rheological equation for the Maxwell element is
1
1
ε«(t ) = σ« + σ
η
E
σ
dε σ 0 ω
=
cos ω t + 0 sin ω t
η
dt
E
(14.16)
Integration of Equation 14.16 between two time limits and noting that ε(o) is not necessarily zero yields
tan δ =
1
τω
Eτ 2 ω 2
E =
1 + ω 2 τ2
(14.17)
1
E11 =
Eτω
1 + ω 2 τ2
(14.18)
where τ = η/E.
B. THE VOIGT ELEMENT
Since, as we saw above, the Maxwell element is not perfect, it seems logical to consider a parallel
arrangement of the spring and the dashpot. This is the so-called Voigt or Voigt–Kelvin element
(Figure 14.7).
The Voigt element has the following characteristics:
• The spring and the dashpot always remain parallel. This means that the strain in each element is the same.
• The total stress supported by the Voigt element is the sum of the stresses in the spring and the dashpot.
σ T = σs + σ d
(14.19)
Thus, the rheological equation for the Voigt element is given by
σ T = Eε + η
Figure 14.7 The Voigt element.
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dε
dt
(14.20)
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POLYMER VISCOELASTICITY
1. Creep Experiment
In a creep experiment, the applied stress is constant; consequently, Equation 14.20 becomes
σ 0 = Eε + η
dε
dt
(14.21)
This is a linear differential equation with solution [integrate between the limits ε(o) = 0 and ε(τ) = ε(t)]:
ε (t ) =
σ0
1 − e − Et η
E
[
σ
ε (t ) = 0 1 − e − t τ
E
[
]
(14.22)
]
or
J (t ) =
ε (t )
= J 1 − e−t τ
σ0
[
]
where
J=
1
= reciprocal of modulus
E
The creep and creep recovery curves for the Voigt elements are shown in Figure 14.8.
Figure 14.8 Creep and creep recovery curves for the Voigt element.
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On the application of a sudden constant stress in a creep experiment, only the spring offers the initial
resistance to deformation; the spring would elongate instantaneously if possible, but its deformation is
constrained by that of the dashpot. Recall that for the Voigt element both the spring and the dashpot
have equal strains. Therefore, the initial total stress is borne by the dashpot. Under the influence of the
constant force, the dashpot begins to flow thus transferring part of the load to the spring. The transfer
of load to the spring results in a concomitant decrease in the stress on the dashpot and hence a decrease
in the strain rate, which is proportional to the magnitude of the stress experienced by the dashpot.
Eventually, the element comes to its equilibrium strain. At this point the strain rate is zero; the resistance
of the dashpot is therefore also zero which means that the entire stress is now supported by the spring.
The equilibrium strain is simply the strain due to the spring (σ0/E). If the load is removed after
equilibrium, the strain decays exponentially.
We note that the Voigt model predicts that strain is not a continuous function of stress; that is, the
element does not deform continuously with the sustained application of a constant stress. The strain
approaches an asymptomatic value given by (σ0 /E). The strain of the element at equilibrium is simply
that of an ideal elastic solid. The only difference is that the element does not assume this strain
instantaneously, but approaches it gradually. The element is shown to exhibit retarded elasticity. In creep
recovery, the Maxwell element retracts instantaneously but not completely, whereas the Voigt element
exhibits retarded elastic recovery, but there is no permanent set.
2. Stress Relaxation Experiment
In a stress relaxation experiment (ε = constant), the rheological equation for the Voigt element reduces to
σ (t ) = Eε
(14.23)
This is essentially Hooke’s law. The Voigt model is not suited for simulating a stress relaxation experiment. The application of an instantaneous strain induces an infinite resistance in the dashpot. It would
require an infinite stress to overcome the resistance and get the dashpot to strain instantaneously. This
is obviously unrealistic.
3. Dynamic Experiment
Now consider the response of a Voigt element subjected to a sinusoidal strain:
ε = ε 0 sin ω t
The stress response of the Voigt element is
σ = σs + σ d
σ s = E ε 0 = Eε 0 sin ω t, and σ d = ηε« = ηω ε 0 cos ω t
(14.24)
σ = E ε 0 sin wt + ηω ε 0 cos wt
Since the sine term is the component in phase with the strain and the cosine term denotes the 90° outof-phase term, then
σ ′ = E ε ′ and σ ′′ = ηω ε ′′
(14.25)
Consequently,
E′ =
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σ′
σ ′′
= E ′′ =
= ηω
ε′
ε ′′
tan δ =
E ′′
E′
tan δ =
ηω
= τω
E
(14.26)
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POLYMER VISCOELASTICITY
Example 14.2: Comment on the physical significance of the quantities measured in dynamic mechanical
(oscillating) experiments.
Solution: To appreciate the physical significance of the quantities measured in oscillatory experiments,
we consider the energy changes in a sample undergoing cyclic deformation. We start by noting that in
viscoelastic and purely viscous materials, the stress and strain are out of phase. Figure E.14.2 shows
stress–strain representation of a viscoelastic material. The oscillatory strain is ε = ε0 sin ωt.
a. Purely elastic body: the work per unit volume is
W = σdε
dε = ε 0 cos ωt d(ωt )
For purely elastic body,
σ = E ε = E ε 0 sin ω t
Therefore, the work done over the first quarter cycle of applied strain is given by
W=
∫
π2
Eε sin ωt ε 0 cos ωt d(ωt )
0
= Eε 02
∫
π2
sin ωt cos ωt d(ωt )
0
Eε 02
2
W=
For the second quarter (i.e., integrating from π/2 to π, the result is exactly the same except that
the sign is negative. Thus for a full cycle, in the case of an elastic body, the energy stored in
the first and third quarter cycles is recovered completely in the second and fourth cycles.
b. Completely viscous flow: in this case,
σ = ηε
W=
2π
∫ ηε ω cos ωt ε
0
0
0
cos ωt d(ωt )
= πηωε 02
For a viscous body, the energy imparted is completely dissipated over the full cycle.
Figure E.14.2 Quantities in oscillatory experiments.
c. Viscoelastic material: here there is a phase angle δ between the stress and strain. Thus,
σ = σ * sin (ωt + δ)
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Therefore,
∫
2π
∫
2π
W = σdε = σ * ε 0 sin (ωt + δ) dos ωt d(ωt )
0
0
Integration of this equation shows that for the elastic component of the work per unit volume,
there is no net energy lost or gained. The viscous component becomes
W = π ε 02 E ′′ = π ε 02 E ′ tan δ
This is the net energy loss through viscous heat generation in material.
Example 14.3: A Voigt element has parameters E = 108 N/m2 and η = 5 × 1010 N · s/m2. Sketch the
creep curve for this element if the imposed constant stress is 108 N/m2.
Solution:
J( t ) =
J=
ε (t )
= J 1 − e −t τ
σ0
(
)
1
= compliance = 10 −8 m 2 N
E
τ = η E = retardation time =
5 × 1010 Ns m 2
10 8 N m 2
= 500 s
J( t ) =
ε( t )
= 10 −8 1 − e − t 500
σo
(
(
E(t ) = σ o 10 −8 1 − e − t 500
)
)
= 1 − e − t 500 since σ o = 10 8
Using this equation a creep curve for several decades of time is shown in Figure E14.3.
Figure E.14.3 Creep curve for Voigt element.
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POLYMER VISCOELASTICITY
Example 14.4: Figure E.14.4 shows the loss modulus–temperature curves for the two materials A and
B. Select either A or B for use as
a. Car tire
b. Engine mount
Explain the basis of your selection.
Figure E.14.4 Energy absorption profiles for two materials.
Solution: The area under the loss modulus–temperature curve is a measure of the damping capacity or
ability to absorb energy of the material. Obviously, A has a higher damping capacity than B.
a. In car tires, temperature buildup contributes to rapid deterioration and wear of tire and loss of
traction. Consequently, material B will be more suitable for use as a car tire since it will absorb
less energy and hence result in less temperature buildup.
b. A critical requirement for an engine mount is the ability to absorb the vibrational loads from
the engine. In this case, a material with the ability to dissipate the vibrational energy as heat
would be preferable; that is, material A.
C. THE FOUR-PARAMETER MODEL
Neither the simple Maxwell nor Voigt model accurately predicts the behavior of real polymeric materials.
Various combinations of these two models may more appropriately simulate real material behavior. We
start with a discussion of the four-parameter model, which is a series combination of the Maxwell and
Voigt models (Figure 14.9). We consider the creep response of this model.
Under creep, the total strain will be due to the instantaneous elastic deformation of the spring of
modulus E, and irrecoverable viscous flow due to the dashpot of viscosity η2, and the recoverable retarded
elastic deformation due to the Voigt element with a spring of modulus E3 and dashpot of viscosity η3 .
Thus, the total strain is the sum of these three elements. That is,
ε(t ) = ε1 + ε 2 + ε 3
ε( t ) =
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σ0 σ0t σ0
+
+
1 − exp − t τ3
E1 η2 E 3
[
(14.27)
]
(14.28)
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POLYMER SCIENCE AND TECHNOLOGY
Figure 14.9 Schematic of the four-parameter model.
where σ0 is the imposed constant stress and τ3 equals η3 /E3 and is referred to as the retardation time.
In creep recovery, say, the load is removed at time t1; the deformation, σ0 /E1, due to the spring of
modulus E1 is recovered instantaneously. This will be followed by the retarded elastic creep recovery
due to the Voigt element given by ε3 or
ε3 =
σ0
1 − exp − t i τ 3
E3
[
]
(14.29)
Only the deformation due to the dashpot of viscosity η2 is retained as a permanent set. The creep and
creep recovery curve of this model is shown in Figure 14.10.
The four-parameter model provides a crude qualitative representation of the phenomena generally
observed with viscoelastic materials: instantaneous elastic strain, retarded elastic strain, viscous flow,
instantaneous elastic recovery, retarded elastic recovery, and plastic deformation (permanent set). Also,
the model parameters can be associated with various molecular mechanisms responsible for the viscoelastic behavior of linear amorphous polymers under creep conditions. The analogies to the molecular
mechanism can be made as follows.
1. The instantaneous elastic deformation is due to the Maxwell element spring, E1 . The primary valence
bonds in polymer chains have equilibrium bond angles and lengths. Deformation from these equilibrium
values is resisted, and this resistance is accompanied by an instantaneous elastic deformation.
2. Recoverable retarded elastic deformation is associated with the Voigt element. This arises from the
resistance of polymer chains to coiling and uncoiling caused by the transformation of a given equilibrium
conformation into a biased conformation with elongated and oriented structures. The process of coiling
and uncoiling requires the cooperative motion of many chain segments, and this can only occur in a
retarded manner.
3. Irrecoverable viscous flow is due to the Maxwell element dashpot η3. This is associated with slippage
of polymer chains or chain segments past one another.
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POLYMER VISCOELASTICITY
Figure 14.10 Creep response of the four-parameter model.
Example 14.5: The constants for a four-parameter model are
E1 = 5 × 108 N m 2 , η2 = 5 × 1010 N ⋅ s m 2 , E 3 = 108 N m 2 , and η3 = 5 × 108 N ⋅ s m 2
For creep and creep recovery experiments calculate:
a. The instantaneous elastic strain
b. The recoverable retarded elastic strain
c. The permanent set
Assume that the creep experiment lasted for 200 s and that the imposed stress is 108 N/m2.
Solution:
a. Instantaneous elastic strain
ε1 =
=
σ0
E1
108 N m 2
= 0.2
5 × 108 N m 2
b. Recoverable retarded elastic strain given by
ε3 =
σ0
η
5 × 108 N ⋅ s m 2
1 − e − t τ3 ; τ 3 = 3 =
=5s
108 N m 2
E3
E3
ε3 =
108
1 − e −200 5
108
[
[
]
]
= 1.0 − 4.25 × 10 −18 = 1.0
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c. Permanent set
ε2 =
=
σ0t
η2
(10
8
)
N m 2 (200 s)
5 × 1010 N ⋅ s m 2
= 0.4
V. MATERIAL RESPONSE TIME — THE DEBORAH NUMBER
A physical insight into the viscoelastic character of a material can be obtained by examining the material
response time. This can be illustrated by defining a characteristic time for the material — for example,
the relaxation time for a Maxwell element, which is the time required for the stress in a stress relaxation
experiment to decay to e–1 (0.368) of its initial value. Materials that have low relaxation times flow easily
and as such show relatively rapid stress decay. This, of course, is indicative of liquidlike behavior. On
the other hand, those materials with long relaxation times can sustain relatively higher stress values.
This indicates solidlike behavior. Thus, whether a viscoelastic material behaves as an elastic solid or a
viscous liquid depends on the material response time and its relation to the time scale of the experiment
or observation. This was first proposed by Marcus Reiner, who defined the ratio of the material response
time to the experimental time scale as the Deborah number, Dn. That is,
Dn =
material response time
experimental time scale (observation time)
(14.30)
A high Deborah number that is a long response time relative to the observation time implies viscoelastic
solid behavior, whereas a low value of Deborah number (short response time relative to the time scale
of experiment) is indicative of viscoelastic fluid behavior. From a conceptual standpoint, the Deborah
number is related to the time one must wait to observe the onset of flow or creep. For example, the
Deborah number of a wooden beam at 30% moisture is much smaller than that at 10% moisture content.
For these materials the onset of creep occurs within a reasonably finite time. At the other extreme, the
Deborah number of a mountain is unimaginably high. Millions of years must elapse before geologists
find evidence of flow. This apparently is the genesis of Marcus Reiner’s analogy (“The mountains flowed
before the Lord” from the Song of Deborah, Book of Judges V).
It must be emphasized, however, that while the concept of the Deborah number provides a reasonable
qualitative description of material behavior consistent with observation, no real material is characterized
by a simple response time. Therefore, a more realistic description of materials involves the use of a
distribution or continuous spectrum of relaxation or retardation times. We address this point in the
following section.
VI.
RELAXATION AND RETARDATION SPECTRA
Real polymers are not characterized by a simple response time. Instead, a distribution or continuous
spectrum of relaxation or retardation times is required for a more accurate description of real polymers.
Many complex models have been proposed to simulate the viscoelastic behavior of polymeric materials.
We discuss two of these models.
A. MAXWELL–WEICHERT MODEL (RELAXATION)
The generalized model consists of an arbitrary number of Maxwell elements in a parallel arrangement
(Figure 14.11).
Consider the generalized Maxwell model in a stress relaxation experiment. The strain in all the
individual elements is the same, and the total stress is the sum of the stress experienced by each element.
Thus,
σ = σ1 + σ 2 + σ 3 + … + σ n −1 + σ n
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(14.31)
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POLYMER VISCOELASTICITY
Figure 14.11 The Maxwell–Weichert model.
The individual stress in each element is given by
σ i = σ 0 e − t τi
(14.32)
This gives the stress relaxation of an individual element under a constant strain ε0 as
σ i (t ) = ε 0 E i e − t τi
(14.33)
where τi = ηi /Ei . For the Maxwell-Reichert model under a constant strain, ε0,
n
∑E e
σ (t ) = ε 0
− t τi
(14.34)
i
i =1
or
E (t ) =
n
∑E e
− t τi
(14.35)
i
i =1
If n is large, the summation in the equation may be approximated by the integral of a continuous
distribution of relaxation times E(r).
E (t ) =
∞
∫ E ( τ) e
−t τ
dτ
(14.36)
0
If one of the Maxwell elements in the Maxwell–Weichert model is replaced with a spring or a dashpot
of infinite viscosity, then the stress in such a model would decay to a finite value rather than zero. This
would approximate the behavior of a cross-linked polymer.
B. VOIGT–KELVIN (CREEP) MODEL
The generalized Voigt element or the Voigt–Kelvin model is a series arrangement of an arbitrary number
of Voigt elements (Figure 14.12). Under creep, the creep response of each individual element is given by
(
ε i (t ) = σ 0 J i 1 − e − t τi
)
(14.37)
or
J i (t ) =
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ε i (t )
= J i 1 − e − t τi
σ0
(
)
(14.38)
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POLYMER SCIENCE AND TECHNOLOGY
Figure 14.12 Voigt-Kelvin model.
1
where Ji = ---and is creep compliance. The response of a series of elements subjected to the same constant
Ei
stress σ0 becomes
J (t ) =
ε (t )
=
σ0
n
∑ J (1 − e )
− t τi
i
(14.39)
i =1
For a large value of n (i.e., n → ∞), the discrete summation in Equation 14.39 may be replaced by an
integration over all the retardation times:
J (t ) =
∫
∞
(
J ( τ) 1 − e − t τ
0
)
dτ
(14.40)
where J(t) is the continuous distribution of retardation times. If the generalized Voigt model is to represent
a linear polymer (viscoelastic liquid), then the modulus of one of the springs must be zero. This element
has infinite compliance and represents a simple dashpot in series with all the other Voigt elements.
Example 14.6: A polymer is represented by a series arrangement of two Maxwell elements with
parameters E1 = 3 × 109 N/m2, t1 = 1 s, E2 = 5 × 105 N/m2, and t2 = 103 s. Sketch the stress relaxation
behavior of this polymer over several decades (at least seven) of time.
Solution:
E r (t ) =
2
∑E e
− t τi
i
i =1
Determining log Er(t) when t varies from 0.01 to 104, a plot of log Er(t) vs. log t is shown in Figure E14.6.
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POLYMER VISCOELASTICITY
Figure E.14.6 Behavior of a 2-component Maxwell-Reichert model.
VII.
SUPERPOSITION PRINCIPLES
In the following sections we discuss the two superposition principles that are important in the theory of
viscoelasticity. The first is the Boltzmann superposition principle, which is concerned with linear viscoelasticity, and the second is time–temperature superposition, which deals with the time–temperature equivalence.
A. BOLTZMANN SUPERPOSITION PRINCIPLE
As discussed earlier for a Hookean solid, stress is a linear function of strain, while for a Newtonian
fluid, stress is a linear function of strain rate. The constants of proportionality in these cases are modulus
and viscosity, respectively. However, for a viscoelastic material the modulus is not constant; it varies
with time and strain history at a given temperature. But for a linear viscoelastic material, modulus is a
function of time only. This concept is embodied in the Boltzmann principle, which states that the effects
of mechanical history of a sample are additive. In other words, the response of a linear viscoelastic
material to a given load is independent of the response of the material to any load previously on the
material. Thus the Boltzmann principle has essentially two implications — stress is a linear function of
strain, and the effects of different stresses are additive.
Let us illustrate the Boltzmann principle by considering creep. Suppose the initial creep stress, σ0,
on a linear, viscoelastic body is increased sequentially to σ1, σ2…σn at times t1, t2…tn, then according
to the Boltzmann principle, the creep at time t due to such a loading history is given by
[
]
[
ε (t ) = J (t ) σ 0 + J (t − t1 ) σ1 − σ 0 + … + J (t − t n ) σ n − σ n −1
]
(14.41)
Here J is compliance, whose functional dependence on time is denoted by the parentheses. The square
brackets denote multiplication. For a continuous loading history, then, the creep is expressed by the
integral:
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ε (t ) =
∫
t
J (t − θ)[σ« (θ)] dθ
(14.42)
0
·
where σ(θ)
describes the stress history. A similar expression can be derived for stress relaxation. In this
case, the initial strain is changed sequentially to ε1 , ε2 , and εn at times t1 , t2 …tn ; then the resultant stress is
[
]
[
σ (t ) = E r (t ) ε 0 + E r (t − t1 ) ε1 − ε 0 + … + E r (t − t n ) t n − t n −1
]
(14.43)
For a continuous strain history, the Boltzmann expression becomes
σ (t ) =
∫
t
E r (t − θ) [ε (θ)] dθ
(14.44)
0
Linear viscoelasticity is valid only under conditions where structural changes in the material do not
induce strain-dependent modulus. This condition is fulfilled by amorphous polymers. On the other hand,
the structural changes associated with the orientation of crystalline polymers and elastomers produce
anisotropic mechanical properties. Such polymers, therefore, exhibit nonlinear viscoelastic behavior.
B. TIME–TEMPERATURE SUPERPOSITION PRINCIPLE
Structural engineering design with engineering materials usually requires that the structures maintain
their integrity for long periods of time. In such designs the elastic modulus of structural components is
an important parameter for relating design stresses to component dimensions. We know, of course, that
the modulus of polymeric materials decreases with increasing time. Therefore, to ensure a safe and
proper design, it is necessary to know the lower limit of modulus. Ideally, the most reliable information
on modulus changes of a polymeric material should be developed over a long period in which test
samples are subjected to conditions comparable to those that will be experienced by the material in realtime service. Accumulation of such long-term data is obviously inconvenient, expensive, and indeed
hardly practical. Consequently, in engineering practice, reliance has to be placed necessarily on shortterm data for the design for long-term applications.
Fortunately for linear amorphous polymers, modulus is a function of time and temperature only (not
of load history). Modulus–time and modulus–temperature curves for these polymers have identical
shapes; they show the same regions of viscoelastic behavior, and in each region the modulus values vary
only within an order of magnitude. Thus, it is reasonable to assume from such similarity in behavior
that time and temperature have an equivalent effect on modulus. Such indeed has been found to be the
case. Viscoelastic properties of linear amorphous polymers show time–temperature equivalence. This
constitutes the basis for the time–temperature superposition principle. The equivalence of time and
temperature permits the extrapolation of short-term test data to several decades of time by carrying out
experiments at different temperatures.
Time–temperature superposition is applicable to a wide variety of viscoelastic response tests, as are
creep and stress relaxation. We illustrate the principle by considering stress relaxation test data. As a
result of time–temperature correspondence, relaxation curves obtained at different temperatures can be
superimposed on data at a reference temperature by horizontal shifts along the time scale. This generates
a simple relaxation curve outside a time range easily accessible in laboratory experiments. This is
illustrated in Figure 14.13 for polyisobutylene. Here, the reference temperature has been chosen arbitrarily to be 25°C. Data obtained at temperature above 25°C are shifted to the right, while those obtained
below 25°C are shifted to the left.
The procedure for such data extrapolation is not arbitrary. The time–temperature superposition
principle may be expressed mathematically for a stress relaxation experiment as
E r (T1, t ) = E r (T2 , t a T )
(14.45)
This means that the effect on the modulus of changing the temperature from T1 to T2 is equivalent to
multiplying the time scale by a shift factor ar which is given by
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POLYMER VISCOELASTICITY
Figure 14.13 Time–temperature superposition for polyisobutylene. (From Tobolsky, A.V. and Catsiff, E., J. Polym.
Sci., 19, 111, 1956. With permission.)
a T = t T t To = τ T τ To = ηT ηTo
(14.46)
where tT is the time required to reach a particular mechanical response (modulus in this case) at
temperature T, and tT0 is the time required to produce the same response at the reference temperature
T0 . An important empirical relation correlating the shift factor with temperature changes has been
developed by Williams, Landel, and Ferry, the so-called WLF equation.3 The WLF equation, which is
valid between Tg and Tg + 100°C, is given by the general expression
log10 a T =
−C1 (T − T0 )
(14.47)
C 2 + T − T0
where T0 is the reference temperature and C1 and C2 are constants to be determined experimentally. The
temperatures are in degrees Kelvin. It is common practice to choose the glass transition temperature,
Tg , as the reference temperature. In this case, the WLF equation is given by
log10 a T =
(
−17.44 T − Tg
51.6 + T − Tg
)
(14.48)
Before shifting the curves to generate the master curve, it is necessary to correct the relaxation modulus
at each temperature for temperature and density with respect to the reference temperature. That is,
(t)
E reduced
=
To ρo
E (t )
T ρ r
(14.49)
ρo = density at To
ρ = density at T
This correction is based on the theory of rubber elasticity, which postulates that the elastic modulus of
rubber is proportional to the absolute temperature T and to the density of the material. It can be argued,
of course, that this correction may only be necessary in the rubbery region, where the predominant
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POLYMER SCIENCE AND TECHNOLOGY
response mechanism is chain coiling and uncoiling. It may not apply in the glassy region, where the
mechanical response is governed essentially by the stretching of bonds and deformation of bond angles,
or the viscous region, which involves chain slippage.
Example 14.7: In a stress relaxation experiment conducted at 25°C, it took 107 h for the modulus of
the polymer to decay to 105 N/m2. Using the WLF equation, estimate how long will it take for the
modulus to decay to the same value if the experiment were conducted at 100°C. Assume that 25°C is
the Tg of the polymer.
Solution:
log10 a T = log
t100 −17.44 (100 − 25)
=
t 25
51.6 + 100 − 25
= −10.33
t100
= 4.66 × 10 −11 h
t 25
t100 = 4.66 × 10 −11 × 10 7 h
= 4.66 × 10 −4 h
VIII.
PROBLEMS
14.1. The following three-parameter model is assumed to simulate the behavior of a certain polymer:
In a creep and creep recovery experiment:
a. What is the rheological equation that describes this model?
b. Sketch the creep and creep recovery curve.
14.2. Calculate the relaxation modulus after 10 s of the application of stress for a polymer represented by
three Maxwell elements in parallel where:
E1 = 105 N/m2, τ1 = 10 s
E2 = 106 N/m2, τ2 = 20 s
E3 = 107 N/m2, τ3 = 30 s
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POLYMER VISCOELASTICITY
14.3. A Voigt–Kelvin model consists of four elements with the following parameters:
Element No.
E(N/m2)
1
2
3
4
5 × 108
1010
5 × 108
108
η (N.s/m2)
5
5
5
5
×
×
×
×
1010
1010
109
1010
In a creep experiment, if the imposed stress is 108 N/m2, determine the strain after 100 s.
14.4. The initial stress on a polymer in a creep experiment is 108 N/m2. This load is increased by 107 N/m2
and 106 N/m2 after 102 and 103 s, respectively. Assuming that the Boltzmann superposition principle
holds for this material, find the strain after 104 s. The creep compliance for the material is given by 10–8
(1 – e–10–4t).
14.5. In a stress relaxation experiment the modulus of polyisobutylene relaxed to 106 N/m2 in 104 h at 0°C.
If it is desired to cut the experimental time to 10 h, use the WLF equation to estimate the temperature
at which the experiment must be conducted. The Tg for PIB is –70°C.
14.6. The relaxation time for a material that obeys the WLF equation at 0°C is 104 s. Its relaxation time at
Tg is 1013 s. What is the relaxation time of this material at 25°C?
14.7. In a forced vibration experiment, the damping peak for polycarbonate occurred at 150°C at a frequency
of 1 Hz. What would be the location of this peak if the frequency were 1000 Hz? Polycarbonate has a
Tg of 150°C.
14.8. A tactic polystyrene has a Tg of 100°C. What are the relative rates of stress relaxation of this polymer
at 150°C and 125°C?
REFERENCES
1.
2.
3.
4.
5.
6.
7.
Reiner, M., Phys. Today, p. 62, January 1964.
Tobolsky, A.V. and Catsiff, E., J. Polym. Sci., 19, 111, 1956.
Williams, M.L., Landel, R.F., and Ferry, J.D, Am. Chem. Soc., 77, 3701, 1955.
Ferry, J.D., Viscoelastic Properties of Polymers, John Wiley & Sons, New York, 1980.
Tobolsky, A.V., Properties and Structures of Polymers, John Wiley & Sons, New York, 1960.
Aklonis, J.J. and MacKnight, W.J., Introduction to Polymer Viscoelasticity, John Wiley & Sons, New York, 1983.
Nielson, L.E., Mechanical Properties of Polymers and Composites, Vol. 1, Marcel Dekker, New York, 1974.
Copyright 2000 by CRC Press LLC
Chapter 15
Polymer Properties and Applications
I.
INTRODUCTION
Many of the products we enjoy every day are derived from polymers even though we may not readily
appreciate this. Polymers influence the way we live, work, and play. The evidence is all around us. There
is no room in our house, no vehicle we ride in, no sports activity we participate in that does not use
products made from polymers; the list of polymer products is virtually endless. In fact, the products are
so diverse, so prevalent, and so ingrained in our lifestyles that we tend to take them for granted.
There are currently more than 40 families of polymers, many of whose uses still go unrecognized
because of their diversity. Most consumers many be familiar with polymers in the form of housewares,
toys, appliance parts, knobs, handles, electrical fixtures, toothbrushes, cups, lids, and packages. But few
are aware that
• the lifelines of communications — television, radio telephone, radar — are based on polymers for
insulation and other vital components;
• foams made from polymers have changed modern concepts of cushioning and insulation;
• polymers are vital to the electrical/electronics industry; and
• the practice of replacing heart valves, sockets and joints, and other defective parts of the body owes it
success to the availability of suitable polymer materials.1
Polymers are true man-made materials that are the ultimate tribute to man’s creativity and ingenuity. It
is now possible to create different polymers with almost any quality desired in the end product — some
similar to existing conventional materials but with greater economic value, some representing significant
property improvements over existing materials, and some that can only be described as unique prime
materials with characteristics unlike any previously known.1
The use of polymers has permeated every facet of our lives. Polymers contribute to meeting our basic
human needs, including food, shelter, clothing, health, and transportation. It is hard to imagine what the
world was like before the advent of synthetic polymers. It is difficult to contemplate what the world,
with all its luxury and comfort, would be like without polymers. In this chapter we discuss the properties
and applications of a number of them. First, however, the structure of and the raw materials for the
polymer industry are briefly highlighted.
II.
THE STRUCTURE OF THE POLYMER INDUSTRY1
The polymer industry is generally made up of a large number of companies. Some of these companies
produce basic raw materials, others process these materials into end-use products, and still others finish
these products in some way that may sometimes make them unrecognizable as polymers. The activities
or functions of these companies are not mutually exclusive; indeed, they frequently overlap. For example,
automotive and packaging, which depend on polymer products for their operations, may themselves be
large processors of polymers. In addition, within the industry itself, polymer materials manufacturers
may also be engaged in processing and finishing operations. In any case, the structure of the polymer
industry can be resolved essentially into the following categories.
A. POLYMER MATERIALS MANUFACTURERS
As has been discussed in Chapter 1, polymers are high-molecular-weight compounds (macromolecules)
built up by the repetition of small chemical units. These units, or repeating units as they are called, are
derived from monomers. Monomers, as we shall see in the Section III, are obtained largely from fractions
of petroleum or gas recovered during the petroleum refining process. The reaction of monomers, referred
to as polymerization, leads to the polymer formation. The polymer materials manufacturer or supplier
is involved in the transformation of monomers or basic feedstocks into polymer materials, which are
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POLYMER SCIENCE AND TECHNOLOGY
then sold in the form of granules, powder, pellets, flakes, or liquids for subsequent processing into
finished products.
A number of polymerization techniques are used in the transformation of monomers into plastics
(Chapter 10). These include bulk, solution, suspension, and emulsion polymerization processes. Each
of these polymerization techniques has its advantages and disadvantages and may be more appropriate
for the production of certain types of polymer materials. For example, bulk polymerization is ideally
suited for making pure polymer products, as in the manufacture of optical-grade poly(methyl methacrylate) or impact-resistant polystyrene, because of minimal contamination of the product. On the other
hand, solution polymerization finds ready application when the end use of the polymer requires a solution,
as in certain adhesives and coating processes.
B. MANUFACTURERS OF CHEMICALS, ADDITIVES, AND MODIFIERS
Very frequently, the polymer material from the materials manufacturer does not go directly to the
processor. There is often an intermediate step that involves the addition of other materials (chemicals,
additives, modifiers) that serve to impart special properties or enhance the qualities of the polymers or
resin. For example, polymers can be integrally colored (with polymers or dyes), made more flexible
(with plasticizers), more heat and light resistant (with stabilizers), or stronger and more impact resistant
(with fiber reinforcements). These modifiers may be supplied by the companies that manufacture the
plastics themselves or by companies that specialize in the production of one or more modifiers.
C. COMPOUNDING/FORMULATING
As we said above, various types of modifiers, chemicals, and additives are compounded with the base resin
or polymer material before processing. The compounding is usually done by the materials manufacturer;
however, this is also performed by a group within the polymer industry. Such companies buy the base resin
from the materials manufacturer and then compound it specially for resale to the processor. The processor
may also buy the base polymer, modifiers, chemicals, and additives for in-house compounding.
D. THE PROCESSOR
The heart of the polymer industry is the processor, who is responsible for turning polymer materials
into secondary products like film, sheet, rod and tube, component parts, or finished end products.
Processors may be classified into three general categories depending on the target market for their
products:
• Custom processors, who do processing on a custom basis for end users
• Proprietary processors, who manufacture polymer products such as housewares and toys for direct sale
to consumers
• Captive processors, who possess in-house production facilities; they are usually manufacturers that
consume large volumes of polymer parts
A number of processing techniques are employed in the plastics industry to shape the plastic material
into the desired end product. In fact, processors are more often categorized on the basis of the type of
processing operations they perform. Polymer processing operations include molding (injection, blow,
rotational, compression, and transfer); extrusion; calendering; thermoforming; and casting (Chapter 11).
E. THE FABRICATOR
Some of the products from the processor are not directly usable by the consumer. The fabricator in the
polymers industry is engaged in turning secondary products such as film, sheet, rods, and tubes into end
products. Using conventional machine tools and simple bending techniques, the fabricator creates
products like jewelry, signs, and furniture from rigid polymers. Similarly, products like shower curtains,
rainwear, inflatables, upholstery, and packaging overwrap are obtained from flexible film and sheeting
by employing various die cutting and sealing methods.
F. THE FINISHER
A number of uses of polymers are not easily recognizable as such because the appearance of polymers
is similar to that of conventional materials. Examples of such polymer products include plastic furniture
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POLYMER PROPERTIES AND APPLICATIONS
415
with integral wood grain patterns and plastic automotive grilles that can be electroplated with a metallic
surface. Production of such polymer products, including the large-volume printing of plastic film and
sheets, is done by the finisher. The finishing of polymers includes the different methods of adding either
decorative or functional surface effects to the polymer product. Color and decorative effects can be added
to polymers prior to and during manufacturing.
Also, polymer parts, whether film and sheeting or rigid products, can be postfinished in a number of
ways. Some companies within the polymers industry specialize in various finishing techniques. On the
other hand, finishing, decorating, and assembly of the polymer end product can be done in-house by the
processor or fabricator.
III.
RAW MATERIALS FOR THE POLYMER INDUSTRY2,3
A finished polymer article may be made entirely from the neat or pure resin. More often, however, it is
necessary to compound the resin with additives to improve its processing behavior and/or enhance
product quality and service performance. In this section we examine, in a global way, the various
feedstocks for the polymers industry. The raw materials can be divided essentially into the base polymers
and the various additives. Polymer additives and reinforcements have been treated in Chapter 9. Therefore, only base polymers are discussed here.
The first plastic made was cellulose nitrate, which is a derivative of cellulose, obtained from wood
pulp. The first truly synthetic polymer material was phenolic resin, which was synthesized from phenol
and formaldehyde derived from coal. Today, the source of organic chemicals for the production of
polymers has shifted from these traditional sources to petroleum and natural gas. Petroleum as a raw
material for organic chemicals (petrochemicals) is relatively cheap, readily available in large tonnages,
and more easily processed than the other main source of organic chemicals — coal.
There are two stages involved in the production of petrochemicals from petroleum. The first is the
separation of petroleum, which is a mixture of hydrocarbons, into various fractions (mainly liquid fuels)
by a process called fractional distillation. The second is the further refinement of certain fractions from
the distillation process to form petrochemicals. Most petrochemicals are derived from three sources:
1. Various mixtures of carbon monoxide and hydrogen, known as synthesis gas, obtained from steam
reforming of natural gas (methane) or, in a few cases, steam reforming of naphtha.
2. Olefins obtained by steam cracking (pyrolysis) of various feedstocks, including ethane, propane–butane
(LPG), distillates (naphtha, gas oil), and even crude oil.
3. Aromatics — benzene, toluene, and xylene (BTX) — obtained from catalytic reforming.
These routes are the sources of the eight building blocks — ammonia, methanol, ethylene, propylene,
butadiene, benzene, toluene, and xylene — from which virtually all large tonnage petrochemicals are
derived. Figure 15.1 represents a simplified version of the production of these chemicals from petroleum
and natural gas. Simplified flow diagrams for the production of some polymers from these basic
petrochemicals are shown in Figures 15.2–15.9.
IV.
POLYMER PROPERTIES AND APPLICATIONS
The rapid growth and widespread use of polymers are due largely to the versatility of their properties.
As we saw in Chapter 3, polymer properties are attributable to their macromolecule nature and the gross
configuration of their component chains as well as the nature and magnitude of interactions between
the constituent chain atoms or groups. The major uses of polymers as elastomers, fibers, and plastics
are a consequence of a combination of properties unique to polymers. For example, the elasticity of
elastomers, the strength and toughness of fibers, and the flexibility and clarity of plastic films reflect
their different molecular organizations. Throughout the discussion in the previous chapters, we have
consistently emphasized the structural basis of the behavior of polymeric materials. We have discussed
the variables necessary to define the mechanical, physical, and other properties of polymers. We now
focus attention on the properties and applications of polymers. We start the discussion with the largevolume polymers — so-called general-purpose or commodity thermoplastics. The selection of polymers
for discussion is based on convenience rather than on a consistent classification scheme.
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POLYMER SCIENCE AND TECHNOLOGY
Figure 15.1 Base petrochemicals from petroleum and gas.
A. POLYETHYLENE
CH2
CH2
(Str. 1)
n
Ethylene may be polymerized by a number of processes to produce different varieties of polyethylene.
The most commercially important of these polymers are low-density polyethylene (LDPE); high-density
polyethylene (HDPE); and, more recently, linear low-density polyethylene (LLDPE) and ultra-highdensity polyethylene (UHDPE).
The first commercial ethylene polymer (1939) was low-density, low-crystalline (branched) polyethylene
(LDPE), which is the largest of the thermoplastics produced in the U.S. LDPE is produced by free-radical
bulk polymerization using traces of oxygen or peroxide (benzoyl or diethyl) and sometimes hydroperoxide
and azo compounds as the initiator. To obtain a high-molecular-weight product, impurities such as hydrogen
and acetylene, which act as chain transfer agents, must be scrupulously removed from the monomer.
Polymerization is carried out either in high-pressure, tower-type reactors (autoclaves) or continuous tubular
reactors operating at temperatures as high as 250°C and at pressures between 1000 and 3000 atm (15,000
to 45,000 psi). The exothermic heat of polymerization (about 25 kcal/mol) is controlled by conducting
the polymerization in stages of 10 to 15% conversion. Solution polymerization of ethylene with benzene
and chlorobenzene as solvents is also possible at the temperatures and pressures employed.
Polyethylene with limited branching, that is, linear or high-density polyethylene (HDPE), can be
produced by the polymerization of ethylene with supported metal–oxide catalysts or in the presence of
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POLYMER PROPERTIES AND APPLICATIONS
417
Figure 15.2 Some polymers from ethylene.
coordination catalysts. The first class of metal–oxide catalyst (Phillips type) consisted of chromium
oxide (CrO3) supported on alumina (Al2O3) or silica–alumina base. Polymerization is carried out at
100 atm and 60 to 200°C in hydrocarbon solvents in which the catalysts are insoluble using either fixedbed, moving bed, fluidized-bed, or slurry processes. The coordination polymerization of ethylene utilizes
Ziegler-type catalysts. These are complexes of aluminum trialkyls and titanium or other transition-metal
halides (e.g., TiCl4). Coordination polymerization of ethylene generally requires lower temperatures and
pressures than those that involve the use of supported metal–oxide catalysts, typically 60 to 70°C and
1 to 10 atm. Linear, low-to-medium-density polyethylene (LLDPE) with shorter chain branches than
LDPE is made also by a low-pressure process (Dow Chemical).
Low-density polyethylene is a partially crystalline solid with a degree of crystallinity in the 50 to
70% range, melting temperature of 100 to 120°C, and specific gravity of about 0.91 to 0.94 (Table 15.1).
Free-radical polymerization of ethylene produces branched polymer molecules. Branches act as defects,
and as such the level of side chain branching determines the degree of crystallinity, which in turn affects
a number of polymer properties. The number of branches in LDPE may be as high as 10 to 20 per 1000
carbon atoms. Branching is of two different types. The first and predominant type of branching, which
arises from intermolecular chain transfer, consists of short-chain alkyl groups such as ethyl and butyl.
The second type of chain branching is produced by intermolecular chain transfer. This leads to longchain branches that, on the average, may be as long as the main chain. High-density polyethylene, on
the other hand, has few side chains, typically 1 per 200 carbon main chain atoms. Linear polyethylenes
are highly crystalline, with a melting point over 127°C — usually about 135°C — and specific gravity
in the 0.94 to 0.97 range.
The physical properties of LDPE depend on three structural factors. These are the degree of crystallinity (density), molecular weight (MW), and molecular weight distribution (MWD). The degree of
crystallinity and, therefore, density of polyethylene is dictated primarily by the amount of short-chain
branching. Properties such as opacity, rigidity (stiffness), tensile strength, tear strength, and chemical
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POLYMER SCIENCE AND TECHNOLOGY
Figure 15.3 Some polymers from propylene.
resistance, which depend on crystallinity, increase as density increases (i.e., the amount of short-chain
branching decreases). On the other hand, permeability to liquids and gases decreases and toughness
decreases with increasing crystallinity.
Copolymerization with polar monomers such as vinyl esters (e.g., vinyl acetate, acrylate esters,
carboxylic acids, and vinyl ethers) can be used to adjust crystallinity and modify product properties.
Ester comonomers provide short-chain branches that reduce crystallinity. For example, LDPE films with
increased toughness, clarity, and gloss have been obtained by the incorporation of less than 7% vinyl
acetate. Films made from ethylene and ethyl acrylate (EEA) copolymers have outstanding tensile strength,
elongation at break, clarity, stress cracking resistance, and flexibility at low temperatures. As we shall
see later, ionomers display extreme toughness and abrasion resistance and improved tensile properties.
With HDPE, control of branching is usually achieved by adding comonomers such as propylene, butene,
and hexene during polymerization.
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POLYMER PROPERTIES AND APPLICATIONS
419
Figure 15.4 Some polymers from butadiene.
The molecular weight of LDPE is typically in the range of 6000 to 40,000. Melt index (MI) is used
as a convenient measure of average molecular weight. Melt index designates the weight (in grams) of
polymer extruded through a standard capillary at 190°C in 10 min (ASTM D 1238). Consequently, melt
index is inversely related to molecular weight. Typical melt index values for LDPE are in the range of
0.1 to 109. As molecular weight increases, tensile and tear strength, softening temperatures, and stress
cracking and chemical resistance increase, while processibility becomes more difficult and coefficient
of friction (film) decreases.
The ratio Mw/Mn is a measure of molecular weight distribution (MWD). The breadth of MWD is
used to evaluate the influence of long-chain branching on the properties of polyethylene. Polyethylene
with a narrow MWD has high impact strength, reduced shrinkage and warpage, enhanced toughness
and environmental stress cracking resistance, but shows a decrease in the ease of processing.
Polyethylene and its copolymers find applications in major industries such as packaging, housewares,
appliances, transportation, communications, electric power, agriculture, and construction. The majority
of LDPE is used as thin film for packaging. Other uses include wire and cable insulation, coatings, and
injection-molded products (Table 15.2).
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POLYMER SCIENCE AND TECHNOLOGY
Figure 15.5 Some polymers from methanol.
Example 15.1: Explain the following observations.
a. LDPE is used mainly as thin film for packaging and sheets while HDPE is used predominantly
in injection molding of crates, pails, tubs, and automobile gas tanks. EVA copolymer containing
a high percentage of the ester comonomer is used in film applications such as disposable
protective gloves.
b. EVA and EEA copolymers are sometimes preferable to conventional LDPE for wire and cable
insulation, particularly for outdoor applications.
Solution:
a. LDPE is used when clarity, flexibility, and toughness are desired. LDPE possesses the desired
combination of low density, flexibility, resilience, high tear strength, and moisture and chemical
resistance, which are characteristics of a good film material. HDPE is used where hardness,
rigidity, high strength, and high chemical resistance are required. EVA copolymers are used in
articles requiring extreme flexibility and toughness and rubbery properties.
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POLYMER PROPERTIES AND APPLICATIONS
Figure 15.6 Some polymers from ammonia.
b. LDPE, on exposure to light and O2, ages with a consequent loss of strength and a reduction in
some other physical properties. It is therefore usual to protect LDPE from the effects of environmental degradation by the addition of carbon black and fillers/stabilizers. Cross-linking of
the polyethylene insulating jacket either through chemical or radiation means can also prevent
environmental aging. For example, incorporation of relatively stable peroxides provides a chemical means of cross-linking polyethylene. The peroxides, which are stable at normal processing
temperatures, decompose and initiate cross-linking in post-processing reactions.
EVA and EEA copolymers as insulating materials show relatively easier acceptance of carbon
black and other stabilizers than does conventional LDPE. They are also easier to cross-link
presumably because of the greater preponderance of tertiary hydrogens (on the chain at branch
points), which are regarded as the probable points of attack during cross-linking. In addition,
the low melting points of the copolymers permit a greater ease of incorporation of peroxides by
minimizing the risk of premature cross-linking during compounding.
B. POLYPROPYLENE (PP)
CH2
(Str. 2)
CH
CH3
n
Polypropylene is the third-largest volume polyolefin and one of the major plastics worldwide
(Table 15.3). The commercial plastic was first introduced in 1957.
Polypropylene is made by polymerizing high-purity propylene gas recovered from cracked gas streams
in olefin plants and oil refineries. The polymerization reaction is a low-pressure process that utilizes
Ziegler–Natta catalysts (aluminum alkyls and titanium halides). The catalyst may be slurried in a
hydrocarbon mixture to facilitate heat transfer. The reaction is carried out in batch or continuous reactors
operating at temperatures between 50 and 80°C and pressure in the range of 5 to 25 atm.
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POLYMER SCIENCE AND TECHNOLOGY
Figure 15.7 Some polymers from benzene.
Polypropylene can be made in isotactic (i-PP), syndiotactic (s-PP), and atactic (a-PP) forms. Ziegler–Natta-type catalysts are used to produce stereoregular polypropylene. Usually 90% or more of the
polymer is in the isotactic form, which is the form with properties of commercial interest. Isotactic
polypropylene is essentially linear, with an ordered arrangement of propylene molecules in the polymer
chain. Unlike polyethylene, isotactic polyethylene does not crystallize in a planar zigzag conformation
due to steric hindrance from the relatively bulky methyl groups. Instead i-PP crystallizes in a helical
form with three monomer units per turn of the helix. Isotactic polyethylene is highly crystalline with a
melting point of 165 to 171°C (Table 15.1). With a density in the range of 0.90 to 0.91 g/cc, polypropylene
is one of the lightest of the widely used commercial thermoplastics.
Polypropylene has excellent electrical and insulating properties, chemical inertness, and moisture
resistance typical of nonpolar hydrocarbon polymers. It is resistant to a variety of chemicals at relatively
high temperatures and insoluble in practically all organic solvents at room temperature. Absorption of
solvents by polypropylene increases with increasing temperature and decreasing polarity. The high
crystallinity of polypropylene confers on the polymer high tensile strength, stiffness, and hardness.
Polypropylene is practically free from environmental stress cracking. However, it is intrinsically less
stable than polyethylene to thermal, light, and oxidative degradation. Consequently, for satisfactory
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POLYMER PROPERTIES AND APPLICATIONS
Figure 15.8 Some polymers from toluene.
processing and weathering, polypropylene must be stabilized by the incorporation of thermal stabilizers,
UV absorbers, and antioxidants.
Polypropylene is used in applications ranging from injection-molded and blow-molded products and
fibers and filaments to films and extrusion coatings. Injection molding uses, which account for about
half of polypropylene produced, include applications in the automotive and appliance fields. Polypropylene can be designed with an integral hinge fabricated into products ranging from pillboxes to cabinet
doors. Extruded polypropylene fibers are used in products such as yarn for carpets, woven and knitted
fabrics, and upholstery fabrics. Nonwoven polypropylene fabrics are used in applications such as carpet
backing, liners for disposable diapers, disposable hospital fabrics, reusable towels, and furniture dust
covers. Polypropylene filaments are employed in rope and cordage applications. Nonwoven polypropylene soft film is suited for overwrap of such products as shirts and hose, while oriented polypropylene
film is used as overwrap of such items as cigarettes, snacks, and phonograph records.
C. POLYSTYRENE
CH2
(Str. 3)
CH
n
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POLYMER SCIENCE AND TECHNOLOGY
Figure 15.9 Some polymers from xylene.
Table 15.1 Properties of Commodity Polymer
Polyethylene
Property
LDPE
HDPE
Specific gravity
0.912–0.94 0.941–0.965
Crystallinity (%)
50–70
80–95
Melting
98–120
127–135
temperature (°C)
Tensile strength
15.2–78.6 17.9–33.1
(MPa)
Tensile modulus
55.1–172
413–1034
(MPa)
Elongation at
150–600
20–130
break (%)
Impact strength
716
0.8–14
(Izod)
(ft-lb/in. notch)
Heat deflection
38–49
60–88
temperature
(°C at 66 psi)
Copyright 2000 by CRC Press LLC
Polystyrene
PVC
Polypropylene
GP-PS
HIPS
SAN
Rigid
Flexible
0.902–905
40–68
165–174
1.04–1.065
—
—
1.03–1.06
—
—
1.07–1.08
—
—
1.30–158
—
—
1.16–135
—
—
29.3–38.6
36.5–54.5
22.1–33.8
62.0–82.7
41.4–51.7 10.3–24.1
1032–1720
24.3–3378 1792–3240 2758–3861 2413–4136
—
500–900
1–2
13–50
1–4
40–80
200–450
0.4–6.0
0.25–0.40
0.5–11
0.35–0.50
0.4–20
—
107–121
75–100
77–93
—
57–82
—
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POLYMER PROPERTIES AND APPLICATIONS
Table 15.2 Typical Applications of Polyethylene
Product
Type
Film extrusion
Extrusion
coating
Wire and cable
insulation
Blow molding
Injection
molding
Typical Applications
LDPE films used in packaging (bags and wrappings for frozen and perishable foods, produce and textile
products); construction cover (moisture barriers and utility covering material); agriculture (greenhouse;
ground cover; tank, pond, and canal liners); garment and stripping bags. HDPE film used for floral
wrapping, grocery bags, snack and food packaging; EVA film used as produce bags, heavy-duty shipping
bags, disposable protective gloves
Laminates of foil, paper and used in milk-type cartons (LDPE) for a variety of foods and drinks. Some
HDPE used in extrusion coatings over flexible foil packages either as a glue layer between paper on
plastics film and aluminum foil or as a heat-seal layer
LDPE used as insulation of high-frequency electrical cables, insulation materials for television, radar,
and multicircuit long-distance telephones
Squeeze bottles, toys, housewares, lids, containers made from LDPE. Blow-molded HDPE used as
containers for bleaches, liquid detergents, milk and other beverages, automobile and truck gas tanks,
crates and pails
Packaging containers, pails and lids, houseware dishpans, and waste baskets, molded furniture seats,
medical labware. HDPE is used in molded structural foam pallets, and crates, underground conduits and
housings
Table 15.3 Typical Applications of Polypropylene
Product Type
Typical Applications
Injection molded
Automotive and appliance fields: distributor caps, radiator fans, accelerator pedals, battery casings,
pillboxes, cabinet doors, bottle crates, jerry cans, cups, plates, file jackets, toys, food and drug
containers
Yarn for carpets, woven sacks and upholstery fabrics, hoses, drinking straws, hypodermic syringes,
reusable towels, overwrap for cigarettes and phonograph records, liners for disposable diapers,
furniture dust covers, geotextiles for road stabilization and erosion control
Extruded
Polystyrene is one of the largest volume thermoplastics. It is a versatile polymer whose principal
characteristics include transparency, ease of coloring and processing, and low cost. It is usually available
in general-purpose or crystal (GP-PS), high impact (HIPS), and expanded grades. Some members of
this family of styrene polymers are copolymers of styrene with other vinyl monomers.
Commercial atactic polystyrene is made by free-radical bulk or suspension polymerization of styrene
with peroxide initiators. The reaction exotherm in bulk or mass polymerization of styrene is controlled
by using a two-stage polymerization process. In the first stage, inhibitor-free styrene is subjected to low
conversion in a stirred tank reactor (prepolymerizer). This is then followed by high conversion in a
cylindrical tower (about 40 ft long by 15 ft diameter) with increasing temperature gradient (Chapter 9).
The pure molten polymer that emerges from the reactor goes through spinnerets or an extruder to provide
the desired finished product.
Polystyrene is a linear polymer that, in principle, can be produced in isotactic, syndiotactic, and
atactic forms. The commercial product or general-purpose polystyrene is atactic and as such amorphous:
isotactic polystyrene is more brittle and more difficult to process than atactic polystyrene. It is therefore
not of commercial interest.
GP-PS is a clear, rigid polymer that is relatively chemically inert. Polystyrene, as produced, has
outstanding flow characteristics and consequently is very easy to process. Its excellent optical properties,
including high refractive index, make it useful in optical applications. However, GP-PS has a number
of limitations, including its brittleness, low heat-deflection temperature (Table 15.1), poor UV resistance,
and susceptibility to attack by a variety of solvents. Polystyrene is sensitive to foodstuffs with high fat
or oil content; it crazes and turns yellow during outdoor exposure.
Many of the problems associated with GP-PS can be alleviated, or at least minimized, through
copolymerization, blending, or proper formulation. For example, polystyrene with enhanced impact
resistance and toughness is produced by the incorporation of butadiene rubber. High-impact polystyrene
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POLYMER SCIENCE AND TECHNOLOGY
Table 15.4 Some Applications of Different Grades of Polystyrene
Processing
Method
Extrusion
Thermoforming
Injection molding
Others
Typical Products
Containers, tubs and trays for food packaging, mirror and picture frames, refrigerator breaker strips,
room dividers, shower doors, glazings, lighting
Refrigerator door liners, signs, horticultural trays, luggage; furniture panels, glove compartment
boxes; meat, poultry, and egg containers; fast-food containers, blister packs, container lids; cookie,
candy, pastry and other food packages
Packaging and nonpackaging disposables; air conditioner grilles; refrigerator and freezer components;
small appliance housings
Consumer electronics such as cassettes, reels, radio, television; and stereo dust covers; business
machine housings, smoke detectors, display racks, toys, disposable tumblers, cutlery, bottles, combs,
brush blocks
(HIPS) is produced commercially by the emulsion polymerization of styrene monomer containing dispersed particles of polybutadiene or styrene–butadiene (SBR) latex. The resulting product consists of a
glassy polystyrene matrix in which small domains of polybutadiene are dispersed. The impact strength
of HIPS depends on the size, concentration, and distribution of the polybutadiene particles. It is influenced
by the stereochemistry of polybutadiene, with low vinyl contents and 36% cis-1,4-polybutadiene providing
optimal properties. Copolymers of styrene and maleic anhydride exhibit improved heat distortion temperature, while its copolymer with acrylonitrile, SAN — typically 76% styrene, 24% acrylonitrile —
shows enhanced strength and chemical resistance. The improvement in the properties of polystyrene in
the form of acrylonitrile–butadiene–styrene terpolymer (ABS) is discussed in Section VII.A.
The expandable grade of styrene homopolymer is used to make foamed products that are beads
generally foamed in place during application. Expandable polystyrene beads may be prepared by the
suspension polymerization of styrene monomer in the presence of a volatile organic blowing or foaming
agent. The foaming agent, such as pentane or hexane, is normally a liquid under polymerization
conditions, but volatilizes during subsequent heating to soften the polymer thus forming a foamed
product. Requirements for various types of products are satisfied by varying bead size, foaming agent
level and composition, polymer molecular weight, and molecular weight distribution. The larger beads,
which generally have the lowest density, find uses in thermal insulation, ceiling tiles, and loose-fill
applications. On the other hand, the smaller beads, which provide better mechanical properties and
surface finish, are employed in custom packaging, insulated drinking cups, and structural and semistructural applications.
The applications for all grades of polystyrene include packaging, housewares, toys and recreational
products, electronics, appliances, furniture, and building and construction insulation (Table 15.4).
D. POLY(VINYL CHLORIDE) (PVC)
CH2
(Str. 4)
CH
Cl
n
Poly(vinyl chloride) is one of the largest volume thermoplastics in the world. It is chemically inert
and versatile, ranging from soft to rigid products that are available at economic costs. PVC is available
in essentially two grades — rigid and flexible.
Commercial grade PVC is produced primarily by free-radical-initiated suspension and emulsion
polymerization of vinyl chloride. Suspension polymerization accounts for over 80% of PVC produced.
Solution and bulk polymerization are also employed to some extent. However, there are difficulties with
bulk polymerization because PVC is insoluble in its monomer and therefore precipitates. In suspension
polymerization, vinyl chloride droplets are suspended in water by means of protective colloids such as
poly(vinyl alcohol), gelatin, or methyl cellulose in pressure vessels equipped with agitators and heat
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POLYMER PROPERTIES AND APPLICATIONS
Table 15.5 Typical Applications of Poly(vinyl chloride)
Area
Piping systems
Building
construction
Transportation
Consumer products
Typical Application
Pressure pipes — water supply and distribution, agricultural irrigation, chemical processing;
nonpressure pipes — drain, waste and vent pipes, sewer systems, conduits for electrical and telephone
cables
Siding, window frames, gutters, interior molding and trim, flooring, wire and cable insulation, wall
coverings, upholstery, shower curtains, refrigerator gaskets
Upholstery, floor mats, auto tops, automotive wire, interior and exterior trim
Footwear, outerwear, phonograph records, sporting goods, toys
removal systems. Polymerization is conducted at temperatures of 40 to 70°C, typically around 60°C.
Higher temperatures can result in minor branching and excessive formation of HCl through dehydrochlorination. Lower temperatures produce a high content of syndiotactic polymers. At a predetermined
end point, unreacted vinyl chloride monomer is stripped from the slurry under vacuum. As a result of
environmental and health concerns, stringent control of the escape of vinyl chloride monomer into the
atmosphere is becoming an important issue.
Poly(vinyl chloride) is partially syndiotactic; it has a low degree of crystallinity due to the presence
of structural irregularities. PVC is relatively unstable to heat and light. Unstabilized PVC undergoes
dehydrochlorination when heated above its Tg (about 87°C) — for example, during melt processing.
This leads to the production of hydrochloric acid, formation of intense color, and deterioration of polymer
properties. Consequently, in practice, a number of ingredients must be added to PVC to enhance thermal
stability and hence improve processing and product performance. Heat stabilizers are the most important
additive. These are generally organometallic salts of tin, lead, barium–cadmium, calcium, and zinc. Other
additives include lubricants, plasticizers, impact modifiers, fillers, and pigments.
The properties of PVC can be modified through chemical modification, copolymerization, and blending. PVC homopolymer contains about 57% chlorine. Chlorinated PVC with the chlorine content
increased to 67% has a higher heat deflection temperature than the homopolymer. This extends the
temperature range over which products can be used, allowing use in residential hot water pipes, for
example. Over 90% of PVC produced is in the form of a homopolymer with the rest as copolymers and,
to a small extent, terpolymers. Flexible film products are obtained by copolymerizing vinyl chloride
with flexible chain monomers such as vinyl acetate and vinylidene chloride. Advantages of copolymers
with small amounts of vinyl acetate over the homopolymer include lower softening point and higher
solubility and, hence, improved processability, enhanced stability, and better color and clarity. The most
commercially important copolymers contain about 13% vinyl acetate and are used for phonograph records
and vinyl floor tiles. Copolymers with vinylidene chloride have better tensile properties than the
homopolymer. They are used in coating applications because of their improved solubility. Copolymers
of vinyl chloride and diethyl fumarate or diethyl maleate (10 to 20% content), while retaining the high
softening temperature of poly(vinyl chloride) homopolymer, have enhanced workability and toughness.
The toughness of PVC can also be improved by blending with high-impact resins like ABS.
As indicated earlier, PVC is available as rigid or flexible resins. Flexible PVC is obtained by
incorporating internal or external plasticizers into PVC. Rigid PVC accounts for about 55% of PVC
used while plasticized or flexible PVC accounts for the remainder. The largest single use of PVC is for
piping systems. The major areas of use and typical applications of PVC are listed in Table 15.5.
Example 15.2: PVC has an advantage over other thermoplastic polyolefins in applications such as
insulation for electrical circuitry in household electronic appliances. Explain.
Solution: Though polar, PVC-like nonpolar thermoplastics can be used as insulation for electrical wires
in low-frequency applications. Prolonged use of household electronic appliances has a tendency to
generate heat. Therefore, these appliances can be a potential source of fire hazard. Unlike other thermoplastic polyolefins, PVC has inherent (built-in) fire retardancy because of its 57% chlorine content. This
reduces the susceptibility to fire outbreak arising from prolonged use of household electronic appliances.
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POLYMER SCIENCE AND TECHNOLOGY
V. OTHER VINYL POLYMERS
As discussed in Chapter 1, olefin polymers are represented by the generalized formula
CH2
(Str. 5)
CH
R
where R may be hydrogen alkyl or aryl, etc. In addition to poly(vinyl chloride) or other halogencontaining polymers, other vinyl polymers are essentially polyolefins in which the R substituents are
bonded to the olefin monomers through an oxygen atom [poly(vinyl esters), poly(vinyl ethers)] or a
nitrogen atom [poly(vinyl amides)]. In this discussion we focus attention on the commercially important
poly(vinyl esters).
A. POLY(VINYL ACETATE) (PVAC)
CH2
(Str. 6)
CH
O
C
CH3
O
n
Poly(vinyl acetate) is the most widely used vinyl ester polymer. It is also the precursor or starting material
for the production of two other polymers that cannot be prepared by direct polymerization because the
starting monomer is unstable. These are poly(vinyl alcohol) and poly(vinyl acetal). The most important
of the latter are poly(vinyl butyral) and poly(vinyl formal).
As a result of its highly exothermic nature, bulk polymerization of vinyl acetate poses problems at
high conversions. The properties of the resulting polymer are susceptible to deterioration due to chain
branching. Therefore, bulk polymerization of vinyl acetate is usually stopped at 20 to 50% conversion.
Thereafter, the unreacted monomer is either distilled off or the polymer precipitated with a suitable
solvent (methanol, ethanol). Poly(vinyl acetate) is manufactured primarily by free-radical-initiated emulsion and, sometimes, solution polymerization.
Only atactic or amorphous poly(vinyl acetate) is currently commercially available. It has a glass
transition temperature, Tg, of 29°C. Consequently, the polymer becomes sticky at temperatures slightly
above ambient. The low-molecular polymers, which are normally brittle, become gumlike when masticated (used in chewing gums). Its adhesive strength is dictated by its water sensitivity.
Poly(vinyl acetate) latex is used in the production of water-based emulsion paints, adhesives, and
textile and paper treatments. Emulsion paints are stable, dry quickly, and are relatively low cost. PVAC
emulsion adhesives are used in labeling and packaging, and as the popular consumer white glue.
Copolymers with dibutyl fumarate, vinyl stearate, 2-ethylhexyl acrylate, or ethyl acrylate are used to
obtain compositions that are softer for emulsion use. As indicated above, a major use of poly(vinyl
acetate) is in the production of poly(vinyl alcohol), which is itself the starting material for poly(vinyl
butyral) and poly(vinyl formal).
B. POLY(VINYL ALCOHOL) (PVAL)
CH2
(Str. 7)
CH
OH
n
Vinyl alcohol is unstable; it is isomeric with acetaldehyde. Therefore, poly(vinyl alcohol) is obtained
indirectly by the alcoholysis of poly(vinyl acetate) in concentrated methanol or ethanol. The reaction is
carried out in the presence of acid or base catalyst; base catalysis is usually faster:
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429
POLYMER PROPERTIES AND APPLICATIONS
CH2
CH2
CH2
CH
CH
CH
O
O
O
C
O
CH2
C
CH3 O
CH2
CH2
alcoholysis
NaOCH3, CH3OH
O
CH
CH
OH
OH
(Str. 8)
poly(vinyl alcohol)
C
CH3
CH2
CH3
poly(vinyl acetate)
CH2
CH2
O,
CH2
CH2
CH2
CH
CH
OH
OH
poly(vinyl alcohol)
CH
CH
O
H
)C
O4
2 2 H 2S
H
(C °C
3
H
0
C
8
H,
EtO
O
CH
n
(CH2)2
CH3
poly(vinyl butyral) (PVB)
HC
HS
HO
2 O
4 , Na
S
2
(Str. 9)
O
CH2
4
CH2
CH
CH
O
O
CH2
n
poly(vinyl formal) (PVF)
Poly(vinyl alcohol) has an atactic chain structure, and it is consequently amorphous. However, it can
be stretched into a crystalline fiber. The small size of the OH groups permits them to fit into a crystal
lattice. Poly(vinyl alcohol) is available in various grades defined by the molecular weight and by the
degree of hydrolysis, which determines polymer water solubility.
The end uses of PVAL include textile and paper treatment and wet-strength adhesives. It is also used
as a polymerization aid such as a thickening and stabilizing agent in emulsion polymerization in cosmetics
and as packaging film requiring water solubility. With their much higher water absorption capacity and
cottonlike feel, formaldehyde-modified poly(vinyl alcohol) fibers, vinal or vinylon fibers, can replace
cotton in applications requiring body contact. These PVAL fibers have good dimensional stability and
abrasion resistance, wash easily, and dry quickly.
Poly(vinyl alcohol) is also used in the manufacture of poly(vinyl butyral) (PVB) and poly (vinyl
formal) (PVF). By far the largest single application of PVB is as an adhesive or plastic interlayer in the
manufacture of laminated safety glass for automotive and aircraft uses. Compared with earlier cellulose
acetate-based laminates, safety glass made from PVB has superior adhesion to glass; it is tough, stable
on exposure to sunlight, clear, and insensitive to moisture. Poly(vinyl formal) is utilized in the manufacture of enamels for heat-resistant electrical wire insulation and in self-sealing gasoline tanks.
Example 15.3: Poly(vinyl butyral) is made by adding butyraldehyde and an acid catalyst, usually sulfuric
acid, to an aqueous solution of poly(vinyl alcohol). For the poly(vinyl butyral) required for safety glass
manufacture, the reaction is stopped at about 75% conversion of the hydroxyl groups. Explain.
Solution: For adequate performance of safety glass in end-use situations, a good bond between the
components of the laminate is imperative. The residual or unreacted hydroxyl groups provide the required
strength and adhesion to glass.
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POLYMER SCIENCE AND TECHNOLOGY
VI.
ACRYLICS
Acrylics comprise a broad array of plastics derived from olefin monomers of the type CH2› CHR in
which the R substituent is a cyamide, carboxylic acid, carboxylic acid ester, or carboxylic acid amide
group. Two ester families, acrylates and methacrylates, constitute the major component of acrylic
polymers. Used singly or as copolymers with monomers containing reactive functional groups, these
monomers provide an array of products ranging from soft, flexible elastomers to hard, stiff plastics and
thermosets, and from highly polar to oleophic resins. Acrylics generally exhibit crystal clarity and
excellent chemical and environmental resistance.
A. POLY(METHYL METHACRYLATE) (PMMA)
CH3
CH2
C
(Str. 10)
n
C
O
O
CH3
The most important member of the acrylic polymers is poly(methyl methacrylate). It is a hard, clear,
colorless, transparent plastic that is usually available as molding and extrusion pellets, reactive syrups,
cast sheets, rods, and tubes.
Poly(methyl methacrylate) for molding or extrusion is produced commercially by free-radical-initiated
suspension or bulk polymerization of methyl methacrylate. To minimize polymerization reaction exotherm and shrinkage, bulk polymerization, which is used in the production of sheets, rods and tubes, is
carried out with a reactive syrup of partially polymerized methyl methacrylate, which has a viscosity
convenient for handling.
Poly(methyl methacrylate) is an amorphous polymer composed of linear chains. The bulky nature of
the pendant group (–O–CO–Me), and the absence of complete stereoregularity makes PMMA an amorphous polymer. Isotactic and syndiotactic PMMA may be produced by anionic polymerization of methyl
methacrylate at low temperatures. However, these forms of PMMA are not available commercially.
Modified PMMA can be obtained by copolymerizing methyl methacrylate with monomers such as
acrylates, acrylonitrile, and butadiene.
Poly(methyl methacrylate) is characterized by crystal-clear light transparency, unexcelled weatherability,
and good chemical resistance and electrical and thermal properties. It has a useful combination of stiffness,
density, and moderate toughness. PMMA has a moderate Tg of 105°C, a heat deflection temperature in the
range of 74 to 100°C, and a service temperature of about 93°C. However, on pyrolysis, it is almost
completely depolymerized to its monomer. The outstanding optical properties of PMMA combined with
its excellent environmental resistance recommend it for applications requiring light transmission and
outdoor exposure. Poly(methyl methacrylate) is used for specialized applications such as hard contact
lenses. The hydroxyethyl ester of methacrylic acid is the monomer of choice for the manufacture of soft
contact lenses. Typical applications of poly(methyl methacrylate are shown in Table 15.6.
Table 15.6 Typical Applications of Poly(methyl methacrylate)
Area
Construction
Lighting
Automotive
Aviation
Household
Others
Copyright 2000 by CRC Press LLC
Typical Applications
Enclosures for swimming pools, shopping malls and restaurants, tinted sunscreens to reduce airconditioning and glare, domed skylights
Lighted signs, luminous ceilings, diffusers, lenses and shields
Lenses, instrument panels, signals and nameplates
Windows, instrument panels, lighting fixture covers, radar plotting boards, canopies
Housings, room dividers, decorating of appliances, furniture, vanities, tubs, counters
Display cabinets and transparent demonstration models in museums, exhibits, and departments stores
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POLYMER PROPERTIES AND APPLICATIONS
B. POLYACRYLATES
CH2
(Str. 11)
CH
n
COOR
Polyacrylates are produced commercially by free-radical-initiated solution and emulsion polymerization
of the appropriate monomer. Unlike for methacrylates, suspension and casting procedures are not feasible
because of the rubber and adhesive nature of higher acrylates.
As shown in Table 15.7, the glass transition temperatures of acrylate polymers are generally below
room temperature. This means that these polymers are usually soft and rubbery.
Solubility in oils and hydrocarbons increases with
Table 15.7 Glass Transition
increasing length of the side group, while polymers
Temperatures of Sample Polyacrylates
become harder, tougher, and more rigid as the size of the
ester group decreases. Polyacrylates have been used in
R
Tg (°C)
finishes and textile sizing and in the production of presMethyl
3
sure-sensitive adhesives. Poly(methyl acrylate) is used
Ethyl
–20
in fiber modification, poly(ethyl acrylate) in fiber modin-Propyl
–44
fication and in coatings, and poly(butyl acrylate) and
n-Butyl
–56
poly(2-ethylhexyl acrylate) are used in paints and adhesive formulation.
Quite frequently, copolymerization is used to optimize the properties of polyacrylates. For example,
copolymers of ethyl acrylate with methyl acrylate provide the required hardness and strength, while
small amounts of comonomers with hydroxyl, carboxyl, amine, and amide functionalities are used to
produce high-quality latex paints for wood, wallboard, and masonry in homes. These functionalities
provide the adhesion and thermosetting capabilities required in these applications. Monomers with the
desired functional groups most often used in copolymerization with acrylates are shown in Table 15.8.
Table 15.8 Functional Comonomers Used with Acrylates
Monomer
Acrylic acid
Functional Group
COOH
Methacrylic acid
Structure
CH2
CH
CH2
C
COOH
COOH
CH3
Itaconic acid
CH2
C
COOH
CH2COOH
Dimethylaminoethyl
methacrylate
NH2
CH2
C
COO
CH2
CH2N
CH3
2-Hydroxyethyl
acrylate
N-Hydroxyethyl
acrylamide
Glycidyl
methacrylate
OH
CH2
O
CH2
CH2
CH
COO
CH2
CHCONH
CH2
C
COO
CH3
From Ulrich, H., Introduction to Industrial Polymers, Oxford Press, Oxford, 1982. .
Copyright 2000 by CRC Press LLC
CH2
CH2
CH2
CH2
CH2
OH
OH
CH2
O
CH2
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POLYMER SCIENCE AND TECHNOLOGY
C. POLYACRYLONITRILE (PAN) — ACRYLIC FIBERS
CH2
(Str. 12)
CH
CN
n
Polyacrylonitrile, like PVC, is insoluble in its own monomer. Consequently, the polymer precipitates
from the system during bulk polymerization. Acrylonitrile can be polymerized in solution in water or
dimethyl formamide (DMF) with ammonium persulfate as the initiator (redox initiation). The polyacrylonitrile homopolymer can be dry spun from DMF directly from the polymerization reactor or wet spun
from DMF into water.
Acrylic fibers are polymers with greater than 85% acrylonitrile content, while those containing 35 to
85% acrylonitrile are known as modacrylic. Acrylic fibers contain minor amounts of other comonomers,
usually methyl acrylate, but also methyl methacrylate and vinyl acetate. These comonomers along with
ionic monomers such as sodium styrene sulfonate are incorporated to enhance dyeability with conventional textile dyes. Modacrylics usually contain 20% or more vinyl chloride (or vinylidene chloride) to
improve fire retardancy.
Polyacrylonitrile softens only slightly below its decomposition temperature. It cannot, therefore, be
used alone for thermoplastic applications. In addition, it undergoes cyclization at processing temperatures.
CH2
CH2
CH
C
CH2
CH
N
C
N
∆
CH2
CH2
CH2
CH
CH
C
C
N
(Str. 13)
N
The resulting polymer, on further heat treatment at elevated temperatures, is a source of graphite
filaments. However, acrylonitrile copolymerized with other monomers finds extensive use in thermoplastic and elastomeric applications. Examples of such copolymers include styrene–acrylonitrile (SAN),
acrylonitrile–butadiene–styrene terpolymer (ABS) and nitrile–butadiene rubber (NBR).
The presence of the highly polar nitrile group (–C;N) in acrylic fibers results in strong intermolecular
hydrogen bonding. This generates stiff, rodlike structures with high fiber strength from which acrylic
fibers derive their properties. Acrylic fibers are used primarily in apparel and home furnishings. They
are more durable than cotton and are suitable alternatives for wool. Typical applications of acrylic fibers
include craft yarns, simulated fur, shirts, blouses, blankets, draperies, and carpets and rugs.
VII.
ENGINEERING POLYMERS
Engineering plastics are high-performance polymers used in engineering applications because of their
outstanding balance of properties. They generally attract a premium price due to their relatively low
production volume and are replacing traditional materials in many engineering applications. For example,
engineering plastics may be used as replacements for metals in automotive and home appliance applications where a high strength-to-weight ratio is an important requirement. As engineering polymers
continue to replace traditional materials in many applications, they are being developed with more
specialized properties so as to be distinctly superior to the displaced material in all significant respects.
Engineering polymers are strong, stiff, tough materials with high thermal stability, excellent chemical
resistance, and good weatherability. They have relatively high tensile, flexural, and impact strengths and
are capable of withstanding a wide range of temperatures. Engineering plastics derive their outstanding
properties mainly from their inherently strong intermolecular forces. The superior properties of engineering polymers can be enhanced by the addition of various types of reinforcements, by blending and
alloy formation, and through chemical modification such as cross-linking. Typical properties of some
engineering polymers are shown in Table 15.9.
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POLYMER PROPERTIES AND APPLICATIONS
Table 15.9 Typical Properties of Some Engineering Polymers
Property
Specific gravity
Tensile strength
(MPa)
Tensile modulus
(MPa)
Flexural strength
(MPa)
Flexural modulus
(MPa)
Impact strength, Izod
(ft-lb/in notch)
Elongation at break
Heat deflection
temperature
(°C at 455 kPa)
a
ABSa
Acetal
1.01–1.04
33.1–43.4
Nylon 6 Nylon 6,6
PC
PPO
PSF
PPS
Polyamide
1.42
65.5–82.7
1.12–1.14
68.9
1.13–1.15
75.8
1.2
65.5
1.06
66.2
1.24
70.3
1.3
65.5
1.36–1.43
72.4–117.9
1586–2275
3585
689
—
2378
2447
2482
3309
2068
55.2–75.8
96.5
34.5
42.1
93.1
93.1
106.2
96.5
131–199
965
1275
2344 2482–2758
2689
3792
3102–3447
1724–2413 2620–2964
3.0–12
1.3–2.3
3.9
2.1
16
1.8–5.0
1.2
1.8–5.0
1.5
5–70
102–107
25–75
124
300
150–185
300
180–240
110
138
20–60
137
50–100
181
20–60
135
8–10
—
High impact grade
A. ACRYLONITRILE–BUTADIENE–STYRENE (ABS)
CH2
CH
CH2
CH
CH
CH2
CH2
(Str. 14)
CH
CN
n
Acrylonitrile–butadiene–styrene resins are terpolymers composed, as the name suggests, of acrylonitrile, butadiene, and styrene. Each component contributes special characteristics to the ultimate properties
of products derived from the resins. Acrylonitrile provides heat and chemical resistance and high strength.
Butadiene acts as the reinforcing agent providing impact strength and toughness even at low temperatures,
while styrene contributes rigidity, easy processability, and gloss. By varying the ratio of the three
components, the designer is provided with ABS resins with a wide range of properties to develop a
variety of products with a well-balanced combination of properties. For example, general-purpose ABS
includes both medium-impact-strength and high-impact-strength grades as well as the low-temperature,
high-impact-strength variety. In addition, there are flame-retardant, structural foam, heat-resistant, lowgloss, and transparent grades.
ABS resins are produced primarily by grafting styrene and acrylonitrile onto polybutadiene latex in
a batch or continuous polymerization process. They may also be made by blending emulsion latexes of
styrene–acrylonitrile (SAN) and nitrile rubber (NBR).
ABS resins consist of two phases: a continuous glassy matrix of styrene–acrylonitrile copolymer and
a dispersed phase of butadiene rubber or styrene–butadiene copolymer. The styrene–acrylonitrile matrix
is ordinarily brittle; however, the reinforcing influence of the rubbery phase results in a product with
greatly improved (high) load-bearing capacity. To optimize properties, it is usually necessary to graft
the glassy and rubbery phases. A variety of ABS resins are produced by varying the ratio of components
and the degree of bonding between the rubbery and glassy phases (graft level).
ABS resins have relatively good electrical insulating properties. They are resistant to weak acids and
weak and strong bases. However, they have poor resistance to esters, ketones, aldehydes, and some
chlorinated hydrocarbons. ABS resins are easily decorated by painting, vacuum metallizing, and electroplating. They are readily processed by all techniques commonly employed with thermoplastics and,
like metals, can be cold-formed. ABS is hygroscopic and therefore requires drying prior to processing.
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POLYMER SCIENCE AND TECHNOLOGY
ABS resins are true engineering plastics particularly suited for high-abuse applications. Injectionmolded ABS is used for housewares, small tools, telephones, and pipe fittings, which are applications
requiring prolonged use under severe conditions. Extruded ABS sheet is used in one-piece camper tops
and canoes. Applications of ABS in automobile and truck machinery include headliners, kick panel,
wheel wells, fender extensions, wind deflectors, and engine covers. Profile-extruded ABS resins are used
in pipes, sewer, well casing, and conduits. Specialty products from ABS include electroplating grades
(automotive grilles and exterior decorative trim); high-temperature-resistant grades (automotive instrument panels, power tool housings); and structural foam grades, which are used in molded parts where
high strength-to-weight ratio is required.
B. POLYACETAL (POLYOXYMETHYLENE — POM)
CH2
(Str. 15)
O
n
Polyoxymethylene (polyacetal) — sometimes known as polyformaldehyde — is the polymer of
formaldehyde. It is obtained either by anionic or cationic solution polymerization of formaldehyde or
cationic ring-opening bulk polymerization of trioxane. Highly purified formaldehyde is polymerized in
the presence of an inert solvent such as hexane at atmospheric pressure and a temperature usually in the
range of –50 to 70°C. The cationic bulk polymerization of trioxane is the preferred method of production
of polyoxymethylene.
Polyoxymethylene is susceptible to depolymerization, or unzipping, under molding conditions. To
improve thermal stability, end capping is essential. The capping of the hydroxyl end groups is achieved
by etherification or, preferably, by esterification using acetic anhydride:
O
cationic
O
O
HOCH2
CH2
O
Ac2O
CH2OH
n
CH3COOCH2
CH2
O
OCH2OOCCH3
n
(Str. 16)
Polyacetal can also be stabilized against degradative conditions by copolymerizing trioxane with
small amounts of ethylene oxide. This introduces a random distribution of –C–C– bonds in the polymer
chain. Hydrolysis of the copolymer with aqueous alkali gives a product with stable hydroxyethyl end
groups. The presence of these stable end groups coupled with the randomly distributed C–C bonds
prevents polymer depolymerization at high temperature.
Polyacetal is a linear, high-molecular-weight polymer with a highly ordered chain structure that
permits an ordered arrangement of chain molecules in a crystalline structure. It is about 80% crystalline,
with a melting point of 180°C. Polyacetal has excellent chemical resistance, has good dimensional
stability due to negligible water absorption, and is insoluble in common solvents at room temperature.
The stiffness, strength, toughness, and creep and fatigue resistance of polyacetals are higher than those
of other unreinforced crystalline thermoplastics. It has good frictional and electrical properties. Products
from polyacetal retain most of these engineering properties over a wide range of useful temperatures
and other end-use conditions. Polyacetals can be processed by the usual molding and extrusion methods.
The relatively advantageous properties of polyacetals have led to applications in a variety of markets,
particularly as a replacement for metals, where it provides enhanced properties and lower costs.
Table 15.10 shows areas and typical applications of polyacetals.
C. POLYAMIDES (NYLONS)
The word nylon is a generic term used to describe a family of synthetic polyamides. Nylons are
characterized by the amide group (–CONH–), which forms part of the polymer main chain (interunit
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POLYMER PROPERTIES AND APPLICATIONS
Table 15.10 Typical Applications of Polyacetals
Area
Automotive
Plumbing
Consumer
Machinery
Typical Applications
Filler necks for gasoline tanks, instrument panels, seat belts, steering columns, window support brackets,
door handles, bearing and gear components, dashboard components, controls, wheel covers, gas caps
Shower heads, shower mixing valves, faucet cartridges, ball cocks
Handles and other hardware items, the bodies of lighters, replaceable cartridges in showers, telephone
components, lawn sprinklers, garden sprayers, stereo cassette cases, spools for video cassettes, zippers
Machinery couplings, small engine starters, pump impellers and housings, fire extinguisher handles, gears
Table 15.11 Nomenclature of Nylons
Monomer(s)
Polymer
H
H2N
(CH2)6
NH2
HOOC
(CH2)4
COOH
Adipic acid
Hexamethylenediamine
N
(CH2)6
H2N
(CH2)6
NH2
HOOC
(CH2)6
Sebacic acid
COOH
(CH2)5
COOH
ω-amino caproic acid
N
C
O
(CH2)4
C
n
N
(CH2)6
H
O
N
C
O
(CH2)6
C
n
Poly(hexamethylene sebacamide), nylon 6,10
H
H2N
O
Poly(hexamethylene adipamide), nylon 6,6
H
Hexamethylenediamine
H
N
O
(CH2)5
C
n
Polycaprolactam, nylon 6
linkage). In terms of chemical structure, nylons may be divided into two basic types: those based on
diamines and dibasic acids (A–A/B–B type); and those based on amino acids or lactams (A–B type).
Nylons are described by a numbering system that reflects the number of carbon atoms in the structural
units. A–B type nylons are designated by a single number. For example, nylon 6 represents polycaprolactam [poly(ω-amino caproic acid)]. A–A/B–B nylons are designated by two numbers, with the first
representing the number of carbon atoms in the diamine and the second referring to the total number
of carbon atoms in the acid (Table 15.11).
Among the nylons, nylon 6,6 and nylon 6 are of the greatest commercial importance and most widely
used. Other commercially useful materials are the higher analogs such as nylon 6,9; 6,10; 6,12; 11; and
12. Nylon 6,6 and nylon 6 are widely used because they offer a good balance of properties at an economic
price. Other nylons command relatively higher prices.
Nylon 6,6 is formed by the step-growth polymerization of hexamethylenediamine and adipic acid. The
exact stoichiometric equivalence of functional groups needed to obtain a high-molecular-weight polymer
is achieved by the tendency of hexamethylenediamine and adipic acid to form a 1:1 salt. This intermediate
hexamethylene diammonium adipate is dissolved in water and then charged into an autoclave. Monofunctional acids such as aluric or acetic acid (0.5 to 1 mol%) may be added to the polymerization mixture
for molecular-weight control. As the temperature is raised, the steam generated is purged by air. The
temperature is raised initially to 220°C, and subsequently to 270 to 280°C when monomer conversion is
about 80 to 90%, while maintaining the pressure of the steam generated at 250 psi. Pressure is then
reduced to atmospheric, and heating is continued until polymerization is totally completed.
A–B type nylons are usually prepared by ring-opening polymerization of a cyclic lactam. Water is
added in a catalytic amount to effect the ring-opening and then removed at higher temperature to
encourage high polymer formation. High-molecular-weight nylon 6 is obtained from the anionic polymerization of ε-caprolactam with a strong base such as sodium hydride. Commercially, nylon 6 is
produced by the hydrolytic polymerization of ε-caprolactam.
As a class, aliphatic polyamides exhibit excellent resistance to wear and abrasion, low coefficient of
friction, good resilience, and high impact strength. Nylons are generally characterized by a good balance
of high strength, elasticity, toughness, and abrasion resistance. They maintain good mechanical properties
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POLYMER SCIENCE AND TECHNOLOGY
Table 15.12 Melting Points and
Moisture Absorption of Some Nylons
Nylon
Tm °C
Water Absorption
(ASTM) D-570)
6,6
6,8
6,9
6,10
6,12
4
6
7
11
12
265
240
226
225
212
265
226
223
188
180
1.0–1.3
—
0.5
—
0.4
—
1.3–1.9
—
—
0.25–0.30
at elevated temperatures — sometimes as high as 150°C — while also retaining low-temperature
flexibility and toughness.
Nylons are sensitive to water due to the hydrogen-bond-forming ability of the amide groups. Water
essentially replaces amide–amide–hydrogen bond with amide–water–hydrogen bond. Consequently,
water absorption decreases with decreasing concentration of amide groups in the polymer backbone
(Table 15.12). Water acts as a plasticizer, which increases toughness and flexibility while reducing tensile
strength and modulus. The absorption of moisture results in a deterioration of electrical properties and
poor dimensional stability in environments of changing relative humidity. Therefore, care must be taken
to reduce the water content of nylon resins to acceptable levels before melt processing to avoid surface
imperfections and embrittlement due to hydrolytic degradation.
The markets for nylon products have been broadened considerably because of the apparent ease with
which polyamides can be modified to produce improved properties for special applications. Modified
nylons of various grades are produced by copolymerization and the incorporation of various additives
(usually in small amounts) such as heat stabilizers; nucleating agents; mold-release agents; plasticizers;
mineral reinforcements (glass fiber/beads, particulate minerals); and impact modifiers. For example, in
applications involving long exposure to temperatures above 75 to 85°C such as automotive under-hood
parts, polyamides have limited use due to their susceptibility to surface oxidation in air at elevated
temperatures and the attendant loss of mechanical properties. Addition of less than 1% copper salt heat
stabilizer permits use of nylons at elevated temperatures. The mechanical properties of nylons depend
largely on their crystallization. Consequently, control of these properties can be achieved partly by
controlling the degree of crystallinity and spherulite size by the use of nucleating agents.
Nylons are used in applications requiring durability, toughness, chemical inertness, electrical insulating properties, abrasion and low frictional resistance, and self-lubricating properties. Table 15.13 lists
markets and typical applications of nylons. In many of these applications, nylons are used as small parts
or elements in subassemblies of the finished commercial article.
The term aramid is used to describe aromatic polyamides, which were developed to improve the heat
and flammability resistance of nylon. Nomex is a highly heat-resistance nylon introduced in 1961 by
DuPont. It is produced by the solution or interfacial polymerization of isophthaloyl chloride and m-phenylenediamine:
Cl
O
O
C
C
Cl
H2N
NH2
O
O
H
H
C
C
N
N
+
n
poly(m-phenyleneisophthalamide) (Nomex)
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(Str. 17)
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POLYMER PROPERTIES AND APPLICATIONS
Table 15.13 Markets and Typical Applications of Nylons
Market
Typical Applications
Automotive/truck
Electrical/electronics
Industrial/machinery
Monofilament/film
coatings
Appliances
Consumer items
Speedometer gears, door lock wedges, distributor point blocks; automotive electrical
system such as connectors, fuse blocks, generator parts, spark wire separators, wire
insulation; monofilament thread for upholstery, license plate bolts and nuts, fuel vapor
canisters, fender extensions, mirrors and grilles; pneumatic tubing and lubrication lines,
fuel and fuel-vent lines
Connector, tie straps, wire coil, bobbins, tuner gears
Lawn mower carburetor components, window and furniture guides, pump parts, power
tools, fans, housings, gears, pulleys, bearings, bushings, cams, sprockets, conveyor
rollers, screws, nuts, bolts, washers
Fishing lines, fish nets, brush bristles, food and medical packaging, coatings for wire
and cable
Refrigerators, dishwashers, ranges, hair dryers and curlers, corn poppers, smoke detectors
Combs, brushes, housewares, buttons, rollers, slides, racquetball racquets
Nomex has a melting point of about 365°C and is virtually nonflammable. It is used in many applications
such as protective clothing and hot gas filtration equipment as a substitute for asbestos.
Another aramid, Kevlar, is the corresponding linear aromatic polyamide obtained from terephthaloyl
chloride and phenylenediamine.
ClCO
COCl + H2 N
NH2
O
O
H
H
C
C
N
N
n
poly(p-phenylene terephthalamide) (Kevlar)
(Str. 18)
Kevlar, which decomposes only above 500°C, provides a fiber material which is as strong as steel at
one-fifth its weight. It is a good substitute for steel in belted radial tires and is used in the manufacture
of mooring lines as well as bulletproof vests and other protective clothing. Fiber-reinforced plastic
composites are also produced from Kevlar fiber. Typical applications of these composites include fishing
rods, golf club shafts, tennis rackets, skis, and ship masts. Significant quantities of Kevlar composites
are used in Boeing 757 and 767 planes.
Example 15.4: Explain the following observations.
a. Major uses of polyacetals are as direct replacements for metals such as brass, cast iron, aluminum,
and zinc in many applications. For example, in the plumbing industry, shower heads, shower
mixing valves, and faucet cartridges molded of acetal homopolymer are replacing brass and zinc
parts.
b. Nylons have good resistance to solvents. However, good solvents for nylon 6,6 and nylon 6 are
strong acids (such as H2SO4, HBr, trichloracetic acid) formic acid, phenols, cresols and perfluoro
compounds.
Solution:
a. The use of polyacetals has a number of property advantages over the use of metals. These include
a good balance of stiffness; light weight; resistance to corrosion, wear, and abrasion; and
dimensional stability. In addition, cost savings result from elimination of metal assemblies, the
reduced number of parts, and the use of low-cost plastic assembly techniques such as welding,
snap fits, and self-threading screws. In other words, the provision of improved properties at
reduced cost permits the use of polyacetals as replacements for metals.
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POLYMER SCIENCE AND TECHNOLOGY
b. The hydrogen bonding capability of nylons is the primary factor controlling their solvent
resistance. The hydrogen bond must be broken or replaced during dissolution. Consequently,
only strong acids, which can protonate the amide nitrogen atom and preclude formation of
hydrogen bonds or other compounds (formic acid, phenols, etc.) that can form hydrogen bonds,
are solvents for nylon 6,6 and nylon 6. These nylons are polyamides with a high content of
amide groups.
Example 15.5: Indicate the polymer property/properties required for the following applications of
nylons:
a.
b.
c.
d.
Gears, bearings, bushings, cams
Screws, nuts, bolts
Food packaging
Coatings for wire and cable insulation.
Solution:
Applications
a. Gears, bearings, bushings, cams
b. Screws, nuts, bolts
c. Food packaging
d. Coatings for wire and cable
Responsible Nylon Properties
High tensile and impact strength; toughness; lubricity; low coefficient of friction
High strength; corrosion resistance
Low permeability to water and air to preserve freshness; high strength; puncture
resistance
Resistance to stress cracking, abrasion, and corrosion; low moisture absorption
D. POLYCARBONATE (PC)
CH3
O
C
O
O
(Str. 19)
C
CH3
n
Polycarbonates are characterized by the carbonate (–O–COO–) interunit linkage. They may be
prepared by interfacial polycondensation of bisphenol A and phosgene in methylene chloride–water
mixture. The resulting hydrogen chloride is removed with sodium hydroxide or, in the case of solution
polymerization, pyridine is used as the hydrogen chloride scavenger. Polycarbonate may also be made
by ester interchange between bisphenol A and diphenyl carbonate.
(Str. 20)
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POLYMER PROPERTIES AND APPLICATIONS
CH3
HO
C
O
OH +
O
C
(Str. 21)
O
CH3
bisphenol A
diphenyl carbonate
CH3
O
C
CH3
O
O
C
n
polycarbonate
Polycarbonate is an amorphous polymer with a unique combination of attractive engineering properties. These include exceptionally high-impact strength even at low temperatures, low moisture absorption, good heat resistance, good rigidity and electrical properties, and high light transmission. It possesses
good dimensional stability (high creep resistance) over a broad temperature range. The transparency of
polycarbonate has led to its use as an impact-resistant substitute for window glass. Polycarbonate,
however, has a limited scratch and chemical resistance. It also has a tendency to yellow under long-term
exposure to UV light. Copolymerization and/or incorporation of additives are used to modify the base
resin for greater creep resistance, UV light performance, flame retardance, and thermal stability. For
example, fire-retardant grades of polycarbonates are made by copolymerizing bisphenol A with tetrabromobisphenol A comonomer, while addition of glass fiber reinforcements greatly extends the level
and range of creep resistance of polycarbonates.
Polycarbonates are processed by all the conventional techniques for processing thermoplastics. The
balanced combination of properties permits polycarbonates to be used in a variety of applications.
Markets for polycarbonates include automotive, construction, electronics, appliances, and lighting, while
typical applications are automobile taillight lenses, lamp housings, bumpers, door and window components, drapery fixtures, furniture and plumbing, business machine housings, machinery housings, telephone parts, glazing signs, and returnable bottles.
E. POLY(PHENYLENE OXIDE) (PPO)
CH3
(Str. 22)
O
CH3
n
Polyphenylene oxide is a large-volume engineering thermoplastic developed in 1956 by General
Electric. It is made by free-radical step-growth oxidative coupling polymerization of 2,6-xylenol, with
copper salts and pyridene as catalysts. Poly(phenylene oxide) homopolymer is difficult to process.
Therefore, the commercial resins, marketed under the trade name Noryl, are modified poly(phenylene
oxides) containing high-impact polystyrene (HIPS). The styrene component of HIPS forms homogeneous
phase with PPO.
Poly(phenylene oxide) is an amorphous thermoplastic material with a low specific gravity, high impact
strength, chemical resistance to mineral and organic acids, good electrical properties, and excellent
dimensional stability at high temperatures. It has exceptionally low water absorption and complete
hydrolytic stability.
Poly(phenylene oxide) resins are available in general-purpose, flame-retardant, glass-reinforced
extrudable, foamable, and specialty grades. In addition to their applications as appliance, electrical, and
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POLYMER SCIENCE AND TECHNOLOGY
business machine housings, PPO resins find use in a variety of pumps, showerheads, and components
for underwater equipment. Platable grades are used for automotive grilles and wheel covers and plumbing
fixtures.
F. POLY(PHENYLENE SULFIDE) (PPS)
(Str. 23)
S
n
Poly(phenylene sulfide), sold by Phillips Chemical Company under the trade name Rylon, is a highly
crystalline aromatic polymer (mp 285°C). It is obtained by the polycondensation of p-dichlorobenzene
and sodium sulfide. The symmetrical arrangement of p-substituted benzene rings and sulfur atoms on
the polymer backbone permits a high degree of crystallization. This, in addition to the extreme resistance
of the benzene ring–sulfur bonds to thermal degradation, confers thermal stability, inherent nonflammability, and chemical resistance on PPS resin.
Even though concentrated oxidizing acids, some amines, and halogenated compounds can affect PPS,
it has no known solvents below 200°C. Poly(phenylene sulfide) is characterized by high stiffness and good
retention of mechanical properties at elevated temperatures. It has high tensile and flexural strengths, which
can be increased substantially by addition of fillers such as glass fibers. PPS has good electrical properties,
low coefficient of friction, and high transparency to microwave radiation. Poly(phenylene sulfide) is
available in grades suitable for injection and compression molding and coating. Principal applications are
in electrical and electronic components and industrial-mechanical uses such as parts for chemical processing
equipment that require high temperature stability, mechanical strength, and chemical resistance.
G. POLYSULFONES
CH3
O
O
C
O
S
CH3
O
n
Polysulfone (Bisphenol A)
(Str. 24)
O
O
S
O
n
Polyethersulfone
(Str. 25)
O
O
O
S
O
n
Polyphenylsulfone
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(Str. 26)
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POLYMER PROPERTIES AND APPLICATIONS
Polysulfones constitute a family of high-performance transparent engineering thermoplastics with
high oxidative and hydrolytic stability and excellent high-temperature properties. They may be prepared
by condensation polymerization of 4,4′-dichlorophenyl sulfone with alkali salt of bisphenol A in polar
solvents like dimethylsulfoxide (DMF) or sulfolane.
CH3
Na+ O-
O
O- Na+
C
+
Cl
CH3
Cl
O
CH3
O
S
(Str. 27)
O
C
O
CH3
S
+ NaCl
O
n
Alternatively, polysulfones may also be synthesized by a Friedel–Crafts reaction of aromatic sulfonyl
chlorides using Lewis acid catalyst:
O
O
SO2Cl
FeCl3
-HCl
O
(Str. 28)
S
O
n
The presence of the diaryl sulfone group with a para oxygen atom confers oxidation resistance, good
thermal stability, and rigidity at high temperatures. The ether linkages provide chain flexibility and,
consequently, impart good impact strength. Polysulfones have good resistance to aqueous mineral acids,
alkali, salt solutions, and oils and greases. They are strong, rigid, tough, amorphous polymers that can
be extruded and injection molded on conventional equipment. Typical properties on some polysulfones
are shown in Table 15.14.
Polysulfones have been used satisfactorily in a wide range of products, including consumer, medical,
automotive, aircraft, aerospace, industrial, electrical, and electronic applications. Like other engineering
thermoplastics, they are replacing metals in a variety of applications because they can be injection molded
into complex shapes at reduced cost since costly machining and finishing operations can be avoided.
Table 15.15 lists some of the applications of polysulfones.
Table 15.14 Typical Properties of Polysulfones
Property
Specific gravity
Tensile strength (MPa)
Tensile modulus (MPa)
Flexural strength (MPa)
Flexural modulus (MPa)
Notched Izod impact strength (ft-lb/in)
Copyright 2000 by CRC Press LLC
Polysulfone
(Bisphenol A)
Polyether
Sulfone
Polyphenyl
Sulfone
1.24
70.3
2482
106.2
2689
1.2
1.37
84.1
2696
128.6
2585
1.6
1.29
71.7
2137
85.5
2275
12.
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POLYMER SCIENCE AND TECHNOLOGY
Table 15.15 Some Applications of Polysulfones
Polysulfone
Typical Applications
Polysulfone
(bisphenol A)
Medical instrumentation; food processing and handling, including microwave ware, coffeemakers,
beverage-dispensing tanks; electrical/electronic applications such as connectors, automotive fuses,
switch housings, television components, structural circuit boards; chemical processing and other
applications, including corrosion-resistant piping, tower packing, pumps, membranes, camera and watch
cases, battery cell frames and housing
High-temperature electrical parts such as connectors, motor components, lamp housings and alternator
insulators; sterilizable medical components; oven windows; aerospace, aircraft and automobile
composite structures
High-temperature coil bobbins, aircraft window reveals, automatic transmission housing, firemen’s
helmets, gas compressor valves, carbon fiber composites, flexible printed circuit boards
Polyether
sulfone
Polyphenyl
sulfone
H. POLYIMIDES
O
O
C
C
N
N
C
C
O
O
n
Polyimide
(Str. 29)
(Str. 30)
O
O
C
C
CH3
N
N
O
C
C
O
O
C
CH3
O
n
(Str. 31)
Polyetherimide (Ulterm)
Polyimides are made by a two-stage process involving an initial polycondensation of aromatic dianhydrides and aromatic diamines to produce a soluble intermediate, polyamic acid, which is then dehydrated
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POLYMER PROPERTIES AND APPLICATIONS
443
at elevated temperatures to form the polyimide. While the intermediate, polyamic acid, is soluble, the cured
or fully imidized polyimide is insoluble and infusible, with essentially the characteristics of a thermoset.
Polyimide molded parts and laminates display high heat, chemical, and wear resistance, with virtually
no creep even at high temperatures. It has high oxidative stability, good electrical insulation properties,
low coefficient of friction, and good cryogenic properties. However, polyimide has inherent structural
weakness arising from imperfections and void formation due to water release in the curing process.
Typical uses of polyimide include electronic applications, sleeve bearings, valve seatings, and compressor vanes in jet engines. Other uses include aircraft and aerospace applications with high performance
requirements. They are used for printed circuit boards in computers and electronic watches for both
military and commercial uses. Polyimides are used in the insulation of automotive parts that require
thermal and electrical insulation, such as wires used in electric motors, wheels, pistons, and bearings.
The thermoplastic variety of polyimides with enhanced melt processability is obtained by combining
the basic imide structure with more flexible aromatic groups such as aromatic ethers or amides. Polyamideimides are produced by condensing trimellitic anhydrides with aromatic diamines, while polyetherimides
are made by the reaction between bisphenol A, 4,4′-methylene dianiline, and 3-nitrophthalic anhydride.
Both polyamide-imide and polyetherimide have high heat distortion temperature, tensile strength,
and modulus. Polyamide-imide is useful from cryogenic temperatures up to 260°C. It is virtually
unaffected by aliphatic and aromatic chlorinated and fluorinated hydrocarbons and by most acid and
alkali solutions. These polymers are used in high-performance electrical and electronic parts, microwave
appliances, and under-the-hood automotive parts. Typical automotive applications include timing gears,
rocker arms, electrical connectors, switches, and insulators.
I.
ENGINEERING POLYESTERS
(Str. 32)
(Str. 33)
Commercially important polyesters are based on polymers with the p-phenylene group in the polymer
chain. In contrast to the low melting, linear aliphatic polyesters, the stiffening action of this group
coupled with the high degree of symmetry results in a high melting point and other important engineering
properties. For example, all commercial polyester fibers are based on terephthalic acid as the primary
building block. Different products are obtained by varying the difunctional alcohols used in polycondensation reaction with this acid. However, the major engineering polyesters are poly(ethylene terephthalate) and poly(butylene terephthalate).
Polyesters are produced commercially by melt polymerization, ester interchanges, and interfacial
polymerization. Commercial poly(ethylene terephthalate) is produced traditionally by two successive
ester interchange reactions. In the first step, dimethyl terephthalate is heated with ethylene glycol at
temperatures near 200°C. This yields an oligomeric dihydroxyethyl terephthalate (x = 1 to 4) and
methanol, which is removed. In the second step, the temperature is increased, leading to polymer
formation, while ethylene glycol is distilled off.
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POLYMER SCIENCE AND TECHNOLOGY
O
O
xCH3OC
COCH3 + 2xHO
HO
CH2CH2
O
CH2CH2
150 – 200°C
OH
O
O
C
C
catalyst
O
CH2CH2
O
H
x
(Str. 34)
+ 2xCH3OH
HO
CH2CH2
O
O
O
C
C
O
CH2CH2
O
H
260 – 300°C
catalyst
x
O
O
C
C
(Str. 35)
O
CH2CH2
O
nx
+ nx HO
CH2CH2
OH
Poly(butylene terephthalate) is a highly crystalline thermoplastic polyester that is manufactured by
condensation polymerization of 1,4-butanediol and dimethyl terephthalate in the presence of tetrabutyl
titanate.
PET and PBT are characterized by high strength, rigidity, and toughness; low creep at elevated
temperatures; excellent dimensional stability; low coefficient of friction; good chemical, grease, oil, and
solvent resistance; minimal moisture absorption; and excellent electrical properties. Reinforcement of
these polymers with glass fibers enhances many of their properties (Table 15.16).
PET has been used for a long time in fiber applications, including apparel, home furnishings, and
tire cord. The fiber applications of PET depend on its outstanding crease resistance, work recovery, and
low moisture absorption. Clothing made from PET fibers exhibit good wrinkle resistance. As a plastic,
PET has been used in the production of films and, more recently, as blow-molded bottles for carbonated
soft drinks. Biaxially oriented PET film is used industrially in magnetic tape, X-ray and photographic
Table 15.16 Properties of Engineering Polyesters
Poly(ethylene terephthalate)
Property
Unfilled
30% Glass Fiber
Reinforced
Specific gravity
Melting temperature (°C)
Tensile strength (MPa)
Tensile modulus (MPa)
Flexural strength (MPa)
Flexural modulus (MPa)
Impact strength (Izod) (ft-lb/in)
Water absorption (24 hr)
1.34–1.39
265
58.6–72.4
2758–4136
96.5–124.1
2413–3102
0.25–0.65
0.1–0.2
1.27
265
158.6
9927
230.9
8962
1.9
0.05
Poly(butylene terephthalate)
Unfilled
30% Glass Fiber
Reinforced
1.31–1.38
224
56.5
1930
82.7–115.1
2275–2758
0.8–1.0
0.08–0.09
1.52
224
117.2–131.0
8962
179.2–200.0
7583–8273
1.3–1.6
0.06–0.08
films, and electrical insulation. Both oriented and unoriented PET films are also used in food packaging
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POLYMER PROPERTIES AND APPLICATIONS
applications such as boil-in-bag food pouches. Applications of 30% glass-fiber-reinforced PET resins
include pump and power-tool housings, sporting goods, and automotive exterior components such as
rearview mirror housings, hinges, and windshield-wiper components.
Automobile applications of PBT include window and door hardware, speedometer frames and gears,
servo pistons, and automobile ignition system components (distributor caps, coil bobbins, and rotors).
In addition, it is used as bases, handles, and housings for small appliances (toasters, cookers, fryers and
irons) as well as small industrial pump housing, impellers, and support brackets, gears, showerhead and
faucet components, and consumer products like buckles, clips, buttons, and zippers.
Example 15.6: Comment on the relative melting points as well as the impact strength and water
absorption of unfilled PET and PBT.
Solution: PBT has four methylene groups compared with two in PET. This confers a greater chain
flexibility on PBT. Consequently, PBT has a lower melting (due to greater susceptibility to thermal
agitation) and a higher impact strength than PET. On the other hand, the hydrocarbon nature of PBT is
greater than that of PET, making PBT more hydrophobic.
CF2
CF2
n
Polytetrafluorethylene (Teflon)
(Str. 36)
F
CF2
C
Cl
n
Polychlorotrifluoroethylene (CTFE)
CH2
(Str. 37)
CF2
n
Poly (vinylidene fluoride) PVDF)
CH2
(Str. 38)
CH
F
n
Poly (vinyl fluoride) (PVF)
(Str. 39)
J. FLUOROPOLYMERS
Fluoropolymers constitute a class of polyolefins in which some or all of the hydrogens are replaced
by fluorine. The structures of some of these polymers are shown above. Fluoropolymers have a broad
range of properties, offering unique performance characteristics. Within this family of polymers are
those with high thermal stability and useful mechanical properties both at high temperatures and at
cryogenic temperatures. Most fluoropolymers are chemically inert and totally insoluble in common
organic solvents. The family of fluoropolymers has extremely low dielectric constants and high dielectric
strength. Most fluoropolymers have unique nonadhesive and low friction properties.
Polytetrafluoroethylene, the most widely used fluoropolymer, is produced by emulsion free-radical
polymerization of tetrafluoroethylene using redox initiators. As a result of its highly regular chain
structure, PTFE is a highly crystalline polymer with high density and melting temperature (mp 327°C).
It is a high-temperature-stable material characterized by low-temperature flexibility and extremely low
coefficient of friction, dielectric constant, and dissipation factor. PTFE exhibits outstanding chemical
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POLYMER SCIENCE AND TECHNOLOGY
inertness, is resistant to attack even by corrosive solvents, and is practically unaffected by water. While
PTFE has a high impact strength, its tensile strength and wear and creep resistance are low relative to
other engineering polymers.
Given its extremely high crystallinity and the associated high melting point, its melt viscosity, and
its low melt flow rates, PTFE cannot be processed by conventional fabrication techniques used for
polymers. Instead, unusual techniques have been developed for shaping polytetrafluoroethylene. Molding
PTFE powders are processed by the two-staged press and sinter methods used in powder metallurgy.
Granular PTFE is first pressed into the desired shape at room temperature and pressure (in the range
2000 to 10,000 psi). The resulting preform is then sintered at a temperature above the crystalline melting
point (360 to 380°C) to obtain a dense, strong, homogeneous product. Better melt processing is achieved
by reducing the crystallinity of PTFE through incorporation of a small concentration of a comonomer
such as hexafluoropropylene. The resulting copolymer retains most of the desirable properties of polytetrafluoroethylene but has a reduced melt viscosity that permits processing by traditional techniques.
Polytetrafluoroethylene is used primarily in applications that require extreme toughness, outstanding
chemical and heat resistance, good electrical properties, low friction, or a combination of these properties.
Principal applications or PTFE are as components or linings for chemical process equipment, hightemperature cable insulation, molded electrical components, tape, and nonstick coatings. Chemical
process equipment applications include linings for pipe, pipe fittings, valves, pumps, gaskets, and reaction
vessels. PTFE is used as insulation for wire and cable, motors, generators, transformers, coils and
capacitors, high-frequency electronic uses, and molded electrical components such as insulators and tube
sockets. Nonstick low friction uses include home cookware, tools, and food-processing equipment.
In addition to polytetrafluoroethylene, several other partially fluorinated polymers are available
commercially. These include poly(chlorotrifluoroethylene), which is also available as a copolymer with
ethylene or vinylidene fluoride, poly(vinyl fluoride), and poly(vinylidene fluoride). Poly(chlorotrifluoroethylene) is a chemically inert and thermally stable polymer, soluble in a number of solvents above
100°C, tough at temperatures as low as –100°C while retaining its useful properties at temperatures as
high as 150°C. Its melt viscosity, though relatively high, is sufficiently low to permit the use of
conventional molding and extrusion processing methods. It is used for electrical insulators, gaskets and
seals, and pump parts.
Poly(vinyl fluoride) is a highly crystalline polymer available commercially as a tough, flexible film
sold under the trade name Tedlar by DuPont. It has excellent chemical resistance like other fluoropolymers, excellent outdoor weatherability, and good thermal stability, abrasion, and stain resistance. It
maintains useful properties between –180°C to 150°C. It is used as protective coatings for materials like
plywood, vinyl, hardboard, metals, and reinforced polyesters. These laminated materials find applications
in aircraft interior panels, in wall covering, and in the building industry.
Poly(vinylidene chloride) is a crystalline polymer (mp 170°C), with significantly greater strength and
creep and wear resistance than PTFE. Poly(vinylidene chloride), which is also available as a copolymer
with hexafluoroethylene, has very good weatherability and chemical and solvent resistance. It is used
primarily in coatings; as a gasket material; in wire and cable insulation; in piping, tanks, pumps and
other chemical process equipment; and in extrusion of vinyl siding for houses.
K. IONOMERS
Ionomers are a family of polymers containing ionizable carboxyl groups, which can create ionic intermolecular cross-links. They are generally copolymers of α-olefins with carboxylic acid monomers that
are partially neutralized by monovalent or divalent cations. A typical ionomer is DuPont Surlyn, which
is a copolymer of ethylene and methacrylic acid partially neutralized with sodium or zinc cations.
CH3
CH2
CH2
CH2
(Str. 40)
C
m
C
O
O- Na+
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n
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POLYMER PROPERTIES AND APPLICATIONS
Table 15.17 Some Commercial Ionomers
Polymer System
Trade Name
Manufacturer
Uses
Poly(ethylene-co-methacrylic acid)
Poly(butadiene-co-acrylic acid)
Perfluorosulfonate ionomers
Perfluorocarboxylate ionomer
Telechelic polybutadiene
Sulfonated ethylene–propylene–diene terpolymer
Surlyn
Hycar
Nafion
Flemion
Hycar
Ionic elastomer
DuPont
BF Goodrich
DuPont
Asahi Glass
BF Goodrich
Uniroyal
Modified thermoplastic
High green strength elastomer
Multiple membrane uses
Chloralkali membrane
Specialty Uses
Thermoplastic elastomer
From Lundberg, R.D., in Encyclopedia of Chemical Technology, 3rd ed., Mark, H.F., Othmer, D.F., Overberger, C.G., and
Seaborg, G.T., Eds., Interscience, New York, 1984. With permission.
For ionomers, the ratio n/m typically does not exceed 10 mol%. The metal ions act as the cross-links
between chains. The ionic interchain forces confer solid-state properties normally associated with a
cross-linked structure on ionomers, but the cross-links are labile at processing temperatures. Consequently, ionomers, like other thermoplastic materials, can be processed on conventional molding and
extrusion equipment.
The preparation of ionomers involves either the copolymerization of a functionalized monomer with
an olefinic unsaturated monomer or direct functionalization of a preformed polymer. Typically, freeradical copolymerization of ethylene, styrene, or other α-olefins with acrylic acid or methacrylic acid
results in carboxyl-containing ionomers. The copolymer, available as a free acid, is then neutralized
partially to a desired degree with metal hydroxides, acetates, or similar salts. The second route for the
preparation of ionomers involves modification of a preformed polymer. For example, sulfonated polystyrene is obtained by direct sulfonation of polystyrene in a homogeneous solution followed by neutralization of the acid to the desired level. Some commercially available ionomers are listed in Table 15.17.
In contrast to homogeneous polymer systems, the pendant ionic groups in ionomers interact or associate,
forming ion-rich aggregates immersed in the nonpolar matrix of polymer backbone (Figure 15.10). The
extent of ionic interactions and, hence, the properties of ionomers, are dictated by the ionic content,
degree of neutralization, type of polymer backbone, and cation.
Ionomers are characterized by outstanding abrasion and oil resistance, toughness, flexibility, good
adhesion, and high transparency. These properties dictate the uses of these polymers. For example, the
high melt viscosity of Surlyn provides good extrusion performance for paper and foil coatings of
multiwall bags for food and drug packaging. Its toughness and abrasion resistance have resulted in
extensive use for covering of golf balls (as a replacement of gutta-percha) and roller-skate wheels. The
high impact strength coupled with its printability has led to the use of Surlyn in the manufacture of
automotive bumper strips and guards. Nafion, with its selective permeability to ions, is used in the
production of chlorine and caustic by electrolysis of salt solutions.
VIII.
ELASTOMERS
As we saw in Chapter 1 elastomers are polymers that are amorphous in the unstretched state and are
above their glass transition temperatures at normal ambient temperatures. They have low glass transition
temperatures, usually in the range –50 to –70°C. Elastomers are composed of irregularly shaped chain
molecules that are held together by a network of cross-links to prevent gross mobility of chains while
permitting local mobility of chain segments. The network of cross-links may be formed by covalent
bonds or may be due to physical links between chain molecules. The process of introducing covalent
cross-links into an elastomer is referred to as vulcanization. Elastomers possess the unique ability to
stretch usually to several times their initial dimensions without rupturing, but retract rapidly with full
recovery on the release of the imposed stress. In the stretched state, elastomers exhibit high strength
and modulus.
A. DIENE-BASED ELASTOMERS
Polymerization of conjugated dienes like butadiene, isoprene, and chloroprenes involves activation of
either or both of the double bonds to give 1,2; 3,4; or 1,4 polymers (Figure 15.11). The residual
unsaturation in the polymer chains provides convenient sites for the introduction of elastomeric network
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POLYMER SCIENCE AND TECHNOLOGY
Figure 15.10 Schematic representation of ionomer. Lines represent nonpolar polymer backbone while + and –
represent metal ions and anions, respectively.
Figure 15.11 Possible polymer structures from the polymerization of conjugated dienes.
of cross-links (vulcanization). Therefore, conjugated dienes are the source of some of the most important
commercially available synthetic rubbers or elastomers. For isoprene and chloroprene, eight arrangements
are theoretically possible: the 1,2 and 3,4 polymers (vinyl polymers) can be isotactic, syndiotactic, or
atactic, while in the 1,4 polymer both cis and trans configurations are possible. In the case of 1,3
butadiene, the 1,2 and 3,4 structures are identical because of the absence of the asymmetrical substituent
group. Both the thermal and physical properties of these polymers are influenced by the relative amounts
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POLYMER PROPERTIES AND APPLICATIONS
of the various structures in the polymer chains. The proportion of each type of structure depends on the
method and conditions of polymerization.
1. Polybutadiene (Butadiene Rubber, BR)
CH2
CH
CH
(Str. 41)
CH2
n
Polybutadiene is second-largest-volume synthetic elastomer, next only to SBR. Polybutadiene can be
produced by free-radical addition polymerization of butadiene. The resulting polymer has predominantly
trans-1,4 units, with only about 20% 1,2 units. As the polymerization temperature is increased, the
proportion of cis-1,4 units increases, while that of 1,2 structure remains essentially unchanged. Butadiene
can also undergo anionic polymerization with lithium or organolithium initiators like n-butyllithium in
nonpolar solvent, such as pentane or hexane. The resulting polymer has a high content of cis-1,4 structure,
which decreases as either higher alkali-metal initiators or more polar solvents are used. High-molecularweight polybutadiene with a high content of trans-1,4 polymers is prepared by solution polymerization
of butadiene using stereo-selective coordination Ziegler–Natta catalysts. Slight changes in catalyst
composition can produce drastic changes in polymer composition.
Like SBR, the principal use of BR is in the production of tires and tire products. BR exhibits good
resilience and abrasion resistance and low heat buildup, which are important requirements for tire
applications. However, in general, BR processes with more difficulty than SBR. Consequently, BR is
blended with SBR and natural rubber in tire manufacturing for improved milling, traction, and wet skid
resistance of tire treads, while BR contributes good resistance to wear and groove cracking, lowered
rolling resistance, and low heat buildup.
2. Styrene–Butadiene Rubber (SBR)
CH2
CH
CH2
CH
CH
CH2
(Str. 42)
Styrene–butadiene rubber is the largest volume synthetic elastomer commercially available. It can be
produced by free-radical emulsion polymerization of styrene and butadiene either at 50 to 60°C (hot
emulsion SBR) or at about 5°C (cold emulsion SBR). The two kinds of SBR have significantly different
properties. The hot emulsion SBR process, which was developed first, leads to a more branched polymer
than the cold emulsion process. Cold SBR has a better abrasion resistance and, consequently, provides
better tread wear and dynamic properties.
SBR may also be produced by anionic solution polymerization of styrene and butadiene with alkyllithium initiator (e.g., butyllithium) in a hydrocarbon solvent, usually hexane or cyclohexane. In contrast
to emulsion SBR, which may have an emulsifier (soap) content of up to 5% and nonrubber materials
sometimes in excess of 10%, solution SBR seldom has more than 2% nonrubber materials in its finished
form. Solution SBR has a narrower molecular weight distribution, higher molecular weight, and higher
cis-1,4-polybutadiene content than emulsion polymerization SBR.
SBR is a random copolymer with a styrene content in the range of 10 to 25%. The presence of styrene
contributes to the good wearing and bonding characteristics of SBR and reduces its price. Also, compared
with polybutadiene rubber alone, strength, abrasion resistance, and blend compatibility are improved.
The butadiene units in SBR are composed approximately of 60 to 70% trans-1,4; 15 to 20% cis-1,4;
and 15 to 20% 1,2 configuration for the polymer at 50°C. Polymers made at lower temperatures have a
higher content of trans-1,4 polybutadiene units. In diene polymerization, high conversion of monomers
or the absence of a chain transfer agent leads to branching due to chain transfer to polymer or reaction
of both double bonds.
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POLYMER SCIENCE AND TECHNOLOGY
The major use of SBR is in the production of tires, particularly passenger-car and light-truck tires,
and in other automotive applications where SBR is blended with other elastomers. These applications
include belts, hoses, seals, and various extruded and molded items. Nontire and nonautomotive uses of
SBR are in industries that require hoses, belts, gaskets, or seals. Others include footwear (shoe soles);
various kinds of solid wheels; roll covers; coated fabrics; and electrical (wire and cable) insulation.
3. Acrylonitrile–Butadiene Rubber (Nitrile Rubber, NBR)
CH2
CH
CH2
CH
CH
CH2
(Str. 43)
CN
Nitrile rubber is a unique elastomer that is a copolymer of butadiene and acrylonitrile. As in SBR
production, butadiene can be copolymerized with 18 to 40% acrylonitrile in either cold or hot freeradical emulsion polymerization. Unlike SBR, NBR is not suitable for tire production, but it is unique
for its excellent oil resistance, which increases with the proportion of acrylonitrile in the copolymer.
Elastomers with high acrylonitrile content (40 to 50%) in addition to high hydrocarbon resistance
generally are also more resistant to abrasion and have lower permeability to gases. Elastomers with
about 20% acrylonitrile content exhibit enhanced resilience that is retained even at low temperatures.
NBR retains its good tensile strength and abrasion resistance even after immersion in gasoline, water,
alcohols, and aromatic solvents. While it has good heat resistance if properly protected with antioxidants,
NBR has poor dielectric properties and ozone resistance.
As result of their excellent oil resistance, nitrile rubbers are used mainly to handle oils, fuels, and
similar chemicals in the form of hoses, tubing, gaskets, seals, O-rings, and gasoline hose. These items
are used in equipment for transportation of all kinds, food processing, and petroleum production. Nitrile
rubbers are also used to enhance the impact strength of polymers such as PVC and ABS. In latex form,
nitrile rubber is used to saturate paper for masking tapes, building papers, and labels.
4. Polyisoprene
CH2
C
CH
(Str. 44)
CH2
n
CH3
Polyisoprene is another widely used commercial synthetic rubber. It is produced by the polymerization
of isoprene (2-methyl-1,3-butadiene) in a hydrocarbon solvent such as n-pentane using Ziegler–Natta
catalyst systems. Out of the eight theoretically possible configurations, only three isomers: cis-1,4; trans1,4; and atactic-3,4 forms have been isolated. Depending on the makeup of the catalyst employed in the
polymerization reaction, a very high content of cis-1,4-polyisoprene can be obtained. The cis-1,4polyisoprene is structurally identical to natural rubber. However, it is cleaner, lighter in color, more
uniform and less expensive to process than natural rubber. The trans-1,4 isomer has limited commercial
use in nonelastomeric applications. It was used originally for covering golf balls, but more recently as
a material for orthopedic splints.
Cis-1,4-polyisoprene, like most diene-based elastomers, has poor resistance to attack by ozone,
gasoline, oil, and organic solvents. It has, however, many of the good properties of natural rubber,
including high resilience, strength, and abrasion resistance. Consequently, it is used mostly in tire making,
usually as a replacement for natural rubber in blends with polybutadiene that are used for making heavyduty truck and bus tires. Other uses of polyisoprene elastomer are in extruded and molded mechanical
goods, footwear, sporting goods, and sealants.
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POLYMER PROPERTIES AND APPLICATIONS
5. Polychloroprene (Neoprene)
CH2
C
CH
CH2
(Str. 45)
n
Cl
Polychloroprene, developed and sold under the trade name Neoprene by DuPont, was the first
commercially successful synthetic elastomer. It is produced by free-radical emulsion polymerization of
chloroprene (2-chloro-1,3-butadiene). The commercial material is mainly trans-1,4-polychloroprene,
which is crystallizable.
Polychloroprenes are noted generally for their good resistance to abrasion, hydrocarbons, sunlight,
oxygen, ozone, gas; weathering characteristics; and toughness. They are more difficult to process than
most synthetic elastomers. Polychloroprene elastomer has a wide range of applications, ranging from
adhesives to wire coverings. All the applications depend on its overall durability. The largest use of
polychloroprene elastomer is in the fabrication of mechanical rubber goods for automotive products;
petroleum production; and transportation, construction, and consumer products. The major uses include
wire and cable coatings, industrial hoses, conveyor belts, diaphragms, seals, gaskets, O-rings, gasoline
tubing, shoe heels, and solid tires. Neoprene latex is used in making gloves, adhesives, and binders.
6. Butyl Rubber
CH3
CH2
CH3
C
CH2
C
CH
(Str. 46)
CH2
CH3
Butyl rubber is a copolymer of isobutylene with a small amount (0.5 to 2.5 mol%) of isoprene, which
provides the unsaturation sites necessary for vulcanization. Butyl rubber is produced by cationic polymerization of isobutylene and chloroprene in methyl chloride in the presence of Friedel–Crafts catalysts
such as aluminum chloride at about –100°C.
Butyl rubber exhibits unusually low permeability to gases and outstanding resistance to attack by
oxygen and ozone. It has excellent chemical inertness, due to the very low residual unsaturation, and
good electrical properties because of its nonpolar saturated nature. Butyl rubber has good tear resistance.
Butyl elastomers can be tailored to have good thermal stability and vibrational damping characteristics.
As a result of its very low gas permeability, butyl rubber is used predominantly in the inner tubes of tires
and inner liners for tubeless tires. Some of the other uses of butyl rubber include sealants, adhesives, hoses,
gaskets, pads for truck cabs, bridge bearing mounts, and other places where vibration damping is important.
B. ETHYLENE–PROPYLENE RUBBERS
CH2
CH2
CH2
CH
(Str. 47)
CH3
ethylene–propylene copolymer (EPM)
CH2
CH2
CH2
CH
CH3
CH
CH
CH
CH2
CH2
CH
C
CH
ethylene–propylene–diene terpolymer (EPDM)
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(Str. 48)
CH3
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POLYMER SCIENCE AND TECHNOLOGY
Copolymerization of ethylene with propylene results in a random, noncrystalline copolymer that is
a chemically inert and rubbery material. EPM is a saturated copolymer that can be cross-linked through
the combination of the free radicals generated by peroxides or radiation. To incorporate sites for
vulcanization, an unsaturated terpolymer can be prepared from ethylene, propylene, and a small amount
(3 to 9%) of a nonconjugated diene (EPDM). The diene is either dicyclopentadiene, ethylidene norbornene, or 1,4-hexadiene. The resulting unsaturated terpolymer can be vulcanized by traditional techniques. Each of the termonomers confers different characteristics on the final elastomer.
Ethylene–propylene elastomers are made by solution polymerization of ethylene and propylene in a
solvent such as hexane using Ziegler–Natta catalysts. EPDM terpolymers can be similarly made by
adding 3 to 9% of any of the above dienes to the monoolefin mixture.
Even though tire use is small, automotive applications are still the largest market for ethylene–propylene elastomers. They are used predominantly in radiator and heater hoses, seals, gaskets, grommets,
and weather stripping. Blends of polypropylene and EPDM are used as material in the manufacture of
car bumpers, fender extensions, and rub strips. Other applications of ethylene–propylene elastomers
include appliance parts, wire and cable insulation, and modification of polyolefins for improved impact
and stress resistance.
O
C. POLYURETHANES
||
Polyurethane is the generic name of polymers with the urethane (–N–C –O–) interunit linkage in the
chain. There are two main synthetic routes for the preparation of linear urethane homopolymer. These
are the condensation reaction between a bischloroformate and a diamine and the addition reaction of a
diisocyanate with diol:
O
Cl
O
C
O
R
O
C
O
Cl +
H2N
R´
NH2
C
O
R
O
O
H
C
N
H
R´
N
n
(Str. 49)
+ 2nHCl
O
C
N
R
N
C
O +
HO
R´
OH
O
H
C
N
R
H
O
N
C
O
R´
O
n
(Str. 50)
Typical diisocyanates include aromatic diisocyanates such as methylenediphenyl isocyanate (MDI) (or
4,4-diphenylmethane dissocyanate) and toluene diisocyanate (TDI). TDI is generally supplied as an
80:20 mixture of 2,4 and 2,6 isomers. Aliphatic diisocyanates include 1,6-hexamethylene diisocyanate
(HMI). Dihydroxyl compounds employed are usually hydroxyl-terminated low-molecular-weight polyesters, polyethers, hydrocarbon polymers, and polydimethylsiloxanes. A typical example is polytetramethylene oxide (PTMO). Cross-linking or chain extension is achieved by using a number of di- and
polyfunctional active hydrogen-containing compounds, the most significant of which are diol, diamines,
and polydroxyl compounds.
Polyurethanes are used in four principal types of products: foams, elastomers, fibers, and coatings.
The majority of polyurethane is used as rigid or flexible foams. However, about 15% is used for elastomer
applications. Production of polyurethane elastomers involves a number of steps. As indicated above, an
intermediate hydroxyl-terminated low-molecular-weight polyester or polyether is prepared. This intermediate is reacted with an isocyanate to form a prepolymer (macrodiisocyanate). The prepolymers are
coupled or vulcanized by adding a diol or diamine:
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453
POLYMER PROPERTIES AND APPLICATIONS
20
C
N
R
N
C
O + HO
(R´)n
OH
(Str. 51)
intermediate
O
C
N
R
H
O
N
C
O
(R´)n
O
O
H
C
N
R
N
C
O
prepolymer (macrodiisocyanate)
O
C
N
R
H
O
N
C
O
(R´)n
O
O
H
C
N
R
N
C
O + HO
prepolymer
O
R´´
O
R´´
OH
diol
O
H
C
N
R
H
O
N
C
O
(R´)n
O
O
H
C
N
R
(Str. 52)
H
O
N
C
O
R´´
O
x
polyurethane
O
C
N
R
H
O
N
C
O
(R´)n
O
O
H
C
N
R
N
C
O + H2 N
prepolymer
H
N
R´´
R´´
NH2
diamine
H
O
H
N
C
N
R
H
O
N
C
O
(R´)n
O
O
H
C
N
R
H
O
H
N
C
N
(Str. 53)
H
R´´
N
x
polyurethane urea
Polyurethane elastomers have high strength; extremely good abrasion resistance; good resistance to
gas, greases, oils, and hydrocarbons; and excellent resistance to oxygen and ozone. Applications include
solid tires, shoe soles, gaskets, and impellers.
D. SILICONE ELASTOMERS
CH3
Si
CH3
(Str. 54)
O
n
Polysiloxanes can be prepared by the hydrolysis of dichlorosilanes such as dimethydichlorosilanes:
(Str. 55)
The process tends to result in the formation of cyclic products, typically trimers and tetramers. Highmolecular-weight elastomers may be obtained by a subsequent base-catalyzed ring-opening polymerization
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POLYMER SCIENCE AND TECHNOLOGY
of these cyclic siloxanes. Cross-linked siloxane elastomers are produced by partially cross-linking these
linear polydimethylsiloxanes through the use of peroxide-based free-radical-initiated process. Alternatively, cross-linking may be effected by the incorporation of trifunctional monomers such as trichlorosiloxanes. To improve the efficiency of vulcanization, unsaturation is introduced into the polymer by
copolymerizing a vinyl-group-containing siloxane. Typically about 10% of the methyl groups in polydimethylsiloxane are replaced by vinylmethylsilanol.
Silicone elastomers are noted for high temperature and oxidative stability, low temperature flexibility,
good electrical properties, and high resistance to weathering and oil. They are used in wire and cable
insulation and surgical implants and as material for gaskets and seals.
E. THERMOPLASTIC ELASTOMERS (TPE)
Thermoplastic elastomers are materials with functional properties of conventional thermoset rubbers and
the processing characteristics of thermoplastics. They do not have the permanent cross-links present in
vulcanized elastomers. Instead, elastomeric properties are the result of rigid-domain structures used to
create a network structure. The domain structure is based on a block copolymer; one block consists of
relatively long, flexible polymer chains (soft segment), while the other block is composed of stiff polymer
molecules (hard segment). The rubber–plastic TPE is a two-phase mixture with a dispersion of the soft
rubbery phase in a continuous glassy plastic matrix (Figure 15.12). Each polymer or major polymer
segment or block has its softening temperature, Ts. The useful temperature range for TPE lies above the
Ts of the elastomeric (soft) phase and below the Ts of the hard phase. Within this temperature range, the
polymer molecules in the soft phase can undergo significant segmental motion. However, this motion
is restricted by the bonds, such as hydrogen bonding, between chemical groups in the hard and soft
phases. The reinforcing action of the hard phase disappears above its softening temperature, and the
TPE behaves as a viscous liquid. Upon cooling, the hard phase resolidifies and the TPE becomes rubbery
again. Similarly, cooling below the Ts of the soft phase changes the material from a rubbery to a hard,
brittle solid. This process is also reversible. It is evident, therefore, that the physical nature of the domain
structure of thermoplastic elastomers permits their reversibility. These elastomers are thermoplastic and
can therefore be fabricated by conventional molding techniques by heating them above the softening
temperature of the hard phase. The advantages and disadvantages of thermoplastic elastomers compared
with conventional cross-linked rubbers are shown in Table 15.18.
A number of different classes of thermoplastic elastomers are currently commercially available. These
include styrenics, polyurethanes, polyesters, polyolefin, and polyamides.
1. Styrene Block Copolymers (Styrenics)
The first and still most commercially important thermoplastic elastomers are those based on A–B–A
block copolymers. A is a high-molecular-weight (50,000 to 100,000) polystyrene hard block, and B is
low-molecular-weight (10,000 to 20,000) diene soft (elastomeric) block such as polybutadiene or polyisoprene or, sometimes, poly(ethylene–butylene).
These types of TPE, known as styrenics, e.g., Shell’s kraton, have a useful temperature range of
–70°C to 100°C. They are viscous melts above 130°C and can be molded and extruded into various
articles. Typical applications include shoe soles, adhesives, and a number of molded products.
2. Thermoplastic Polyurethane Elastomers (TPUs)
Thermoplastic polyurethane elastomers are linear copolymers of the (AB)n type. They are composed of
one block of polymer chain consisting of a relatively long, flexible “soft segment” derived from hydroxyterminated polyester, polyether, or polyalkene. The second copolymer block is composed of a highly
polar, rather stiff block (hard block) formed by the reaction of diisocyanates with low-molecular-weight
diol or diamine chain extender. Intermolecular hydrogen bonding between the –NH group (proton donor)
and the carbonyl group or ether oxygens (proton acceptor) results in a domain structure in which the
hard blocks restrict the movement of the soft segments. This provides resistance to deformation when
these elastomers are stretched.
Thermoplastic polyurethanes exhibit good abrasion and mar resistance. They are stable to attack by
oxygen and hydrocarbons, but are sensitive to moisture and acidic and basic solutions, which can result
in hydrolytic chain scission. They are used as coatings and adhesives and in footwear and automotive parts.
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POLYMER PROPERTIES AND APPLICATIONS
455
Figure 15.12 Thermoplastic elastomers are composed of a two-phase system either of a block copolymer (top)
or a rubber–plastic mixture (bottom).
3. Polyolefin Blends
Olefinic thermoplastic elastomers are block copolymers or blends of polyolefins — commonly, polypropylene, which forms the hard crystalline block, and another olefin block, most commonly ethylene or
EPDM. Some less common soft segments include natural rubber, nitrile rubber, and EVA. Olefinic
thermoplastic elastomers exhibit better processability than neoprene and have excellent resistance to
oils. Therefore, they offer attractive replacements for neoprene in oil-resistant wire and cable insulation.
4. Thermoplastic Copolyesters (COPE)
Thermoplastic copolyesters are condensation block copolymers based on a crystalline polyester hard
segment and an amorphous long-chain polyether soft segment. The hard segment, for example, formed
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POLYMER SCIENCE AND TECHNOLOGY
Table 15.18 Advantages and Disadvantages of Thermoplastic Elastomers
Advantages
TPEs require little or no compounding since they are fully
formulated and are ready for use as received. Offer rapid, efficient,
and economical means of fabricating rubber articles.
Processing of TPEs is cheaper and simpler requiring fewer steps
than does a thermoset rubber. A single step is required to shape
TPEs into final article, whereas thermoset rubbers require mixing
and vulcanization in addition to the shaping step.
Disadvantages
Most TPEs may require drying before processing
TPEs melt at elevated temperatures; consequently,
they are unsuitable for applications requiring even
brief exposures to temperatures above their melting
points. Many thermoset rubbers can withstand such
a short exposure.
Processing times for TPEs are shorter since the long cycle time
required for vulcanization is eliminated. Cycle time is typically
seconds instead of the minutes required for thermoset rubbers.
This enhances productivity and reduces costs.
Being thermoplastic, TPE scrap generated during processing can
be recycled without any loss in functional properties. In contrast,
thermoset elastomers are not recyclable.
As a result of the fewer processing steps and shorter cycle time,
lower energy is consumed in TPE processing than processing of
thermoset rubbers.
It is easier to obtain finished parts having closer tolerances with
TPEs because of consistent material base and greater ease and
control of processing.
Elastomers are generally purchased on weight basis but used on
volume basis. Consequently, TPEs offer cost advantages since
they normally have lower specific gravity than thermoset rubbers.
From Heineck, D.W. and Rader, C.P., Plast. Eng., 45(3), 87, 1987. With permission.
by the reaction of terephthalic acid and butanediol, and the soft segment, such as poly(tetramethylene
ether glycol), are joined by an ester linkage. Thermoplastic copolyesters, for example, DuPont Hytrel,
are durable, high-performance thermoplastic elastomers that exhibit good resistance to abrasion and
hydrocarbons. They are, however, susceptible to attack by moisture, acids, and bases at elevated temperatures. Applications of thermoplastic copolyesters include wire and cable insulation, gaskets, seals,
hoses, and automotive parts.
5. Thermoplastic Polyamides
Thermoplastic polyamides are a relatively new class of block-copolymer thermoplastic elastomers. They
have amide linkages between the hard and soft segments. This class of thermoplastic elastomers has the
highest performance and is the most expensive of the TPEs.
Example 15.7: Currently available thermoplastic elastomers cannot be used in applications with use
temperatures above 170°C. Comment on this statement.
Solution: The thermoplastic elastomers that are currently available are used in numerous applications
requiring the elastomeric properties of conventional thermoset rubbers. These include nontire automotive
applications (fascia, hoses, gaskets, seals, bushings, protective boots, weather stripping); mechanical
rubber goods (components of modern appliances, including dishwashers, power drills, telephone, and
electronic equipment); architectural and construction uses (window glazing systems, seals for windows
and doors, primary electrical insulation and jacketing for cables, and foamed seals and tapes); and use
in rubber articles in contact with food and in medical applications. However, the upper limit of the useful
temperature range of thermoplastic elastomers is set by the softening or melting temperature of the hard
segment. Above this temperature, the TPE loses its elastomeric properties and becomes a viscous liquid.
Thermoplastic elastomers that are currently available have hard segments, with melting or softening
temperatures much below 170°C.
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POLYMER PROPERTIES AND APPLICATIONS
IX.
THERMOSETS
The principal feature that distinguishes thermosets and conventional elastomers from thermoplastics is
the presence of a cross-linked network structure. As we have seen from the above discussion, in the case
of elastomers the network structure may be formed by a limited number of covalent bonds (cross-linked
rubbers) or may be due to physical links resulting in a domain structure (thermoplastic elastomers). For
elastomers, the presence of these cross-links prevents gross mobility of molecules, but local molecular
mobility is still possible. Thermosets, on the other hand, have a network structure formed exclusively
by covalent bonds. Thermosets have a high density of cross-links and are consequently infusible,
insoluble, thermally stable, and dimensionally stable under load. The major commercial thermosets
include epoxies, polyesters, and polymers based on formaldehyde. Formaldehyde-based resins, which
are the most widely used thermosets, consist essentially of two classes of thermosets. These are the
condensation products of formaldehyde with phenol (or resorcinol) (phenoplasts or phenolic resins) or
with urea or melamine (aminoplastics or amino resins).
A. PHENOLIC RESINS
Phenolic resins, introduced in 1908, are formed by either base- or acid-catalyzed addition of formaldehyde to phenol to give ortho- and para-substituted products. The nature of these products depends largely
on the type of catalyst and the mole ratio of formaldehyde to phenol. In resole formation, excess
formaldehyde is reacted with phenol under basic conditions. The initial reaction products are ortho- and
para-substituted mono-, di-, and trimethylolphenols:
OH
CH2OH
OH
OH
CH2OH
(Str. 56)
+ CH2O
CH2OH
OH
HOCH2
CH2OH
CH2OH
When heated, methylol phenols, condense either through methylene or methylene oxide linkages to give
a low-molecular-weight prepolymer called resole, which is soluble and fusible and contains alcohol groups:
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POLYMER SCIENCE AND TECHNOLOGY
OH
OH
CH2
OH
CH2
CH2
CH2OH
OH
CH2OH
HOOH2
CH2OH
OH
+
CH2OH
CH2OH
CH2
OH
(Str. 57)
CH2OH
CH2OH
CH2OH
resole
When resoles are heated at elevated temperatures, under basic, neutral, or slightly acidic conditions,
condensation of large numbers of phenolic nuclei takes place resulting in a high-molecular-weight crosslinked network structure.
Formation of novolak involves an acid-catalyzed reaction of formaldehyde with excess phenol (i.e.,
formaldehyde-to-phenol mole ratio less than 1). The initial methylol phenols condense with the excess
phenol to form dihydroxydiphenyl methane, which undergoes further condensation yielding low-molecular-weight prepolymer or novolak. Unlike resoles, novolaks do not contain residual methylol groups.
They are fusible and insoluble.
OH
OH
OH
OH
CH2OH
+ CH2O
(Str. 58)
+
CH2OH
OH
OH
CH2
CH2
CH2
OH
HO
CH2
CH2
OH
OH
CH2
novalak
When novolaks are heated with additional paraformaldehyde or hexamethylene tetramine to raise the
formaldehyde-to-phenol ratio above unity, high-molecular-weight cross-linked network structure is
formed.
The largest use of phenol–formaldehyde resins is in plywood manufacture. Other applications include
lacquers and varnishes, cutlery handles, and toilet seats. Molded parts are used in distributor caps, fuse
boxes, and other electrical outlets because of the superior dimensional stability and electrical properties
of PF resins. Decorative laminates from PF resins are used for countertops and wall coverings, while
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POLYMER PROPERTIES AND APPLICATIONS
459
industrial laminates are used for electrical parts, including printed circuits. Other industrial applications
of phenolics based on their excellent adhesive properties and bond strength include brake linings, abrasive
wheels and sandpaper, and foundry molds.
B. AMINO RESINS
Urea–formaldehyde (UF) resins are obtained by a two-staged polymerization process. The first stage
involves the reaction of formaldehyde with urea at a formaldehyde-to-urea (F/U) mole ratio equal to or
greater than one under slightly alkaline conditions. This results in the formation of mono- and dimethylol
urea and, possibly, trimethylol urea, depending on the F/U ratio (Figure 15.13A). The next stage involves
condensation of these methylol ureas, usually under acidic conditions. Depending on the extent of
reaction, the condensation reactions lead ultimately to a cross-linked product (Figure 15.13B). The
production of melamine–formaldehyde resins, like UF resins, involves initial methoylation reaction
followed by the formation of a rigid network structure (Figure 15.13C,D).
In contrast to phenolic resins, amino resins are clear and colorless. They are harder and have higher
strength but lower moisture and heat resistance than phenolics. Melamine–formaldehyde resins have
better moist-heat aging resistance than UF resins, but are more costly. Like phenolics, aminoplasts can
be used to improve the shrink and crease properties, fire retardance, and water repellency of textiles and
the wet and bursting strength of paper. UF resins are used in molding and in laminating applications.
However, a greater part of UF resins are used for adhesive applications, particularly where the darker
color of phenolics may be objectionable and the relatively inferior durability of UF resins does not pose
Figure 15.13 Two-staged process for the formation of network structure by UF resin (A and B) and MF resin
(C and D).
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POLYMER SCIENCE AND TECHNOLOGY
a serious problem. A typical example is in the production of interior grade plywood. UF resins are also
used in the manufacture of electrical switches and plugs. Typical applications of MF resins include highquality decorative dinnerware, laminated counter-, cabinet, and tabletops; and electrical fittings. Formaldehyde-based resins may be used neat or compounded with various additives such as wood flour to
improve strength properties and reduce cost.
C. EPOXY RESINS
Epoxy resins are complex network polyethers usually formed in a two-staged process. The first stage
involves a base-catalyzed step-growth reaction of an excess epoxide, typically epichlorohydrin with a
dihydroxy compound such as bisphenol A. This results in the formation of a low-molecular-weight
prepolymer terminated on either side by an epoxide group.
(Str. 59)
The prepolymer may be a viscous liquid, generally for commercial epoxies, or a hard and tough solid
depending on the value of n. Other hydroxyl-containing compounds such as resorcinol, glycol, and
glycerol can also be used but commercial epoxy resins are based on bisphenol A. In the second stage,
a cross-linked network structure is formed by curing the prepolymer with active hydrogen-containing
compounds. These curing agents include polyamines, polyacids and acid anhydrides, polyamides, and
formaldehyde resins. Amines, preferably liquid amines like triethylene diamine, effect cure of the
prepolymer by ring-opening of the terminal epoxide groups.
O
O
RNH2 + CH2
OH
CH
RNHCH2
CH2
CH
CH
(Str. 60)
OH
R
N
CH2
CH
CH2
CH
OH
Epoxy resins are very versatile materials with a wide range of applications. They are tough and
flexible and have high tensile, compressive, and flexural strengths. Epoxy resins have excellent chemical
and corrosion resistance and good electrical insulation properties. They can be cured over a wide range
of temperatures with very low shrinkage. Epoxy resins have outstanding adhesion to a variety of
substrates, including metals and concrete. Consequently, the major uses of epoxy resins are in protective
coating applications. Other uses of epoxies include laminates and composites. Potting, encapsulation,
and casting with epoxy resins are common procedures in the electrical and tooling industries.
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POLYMER PROPERTIES AND APPLICATIONS
D. NETWORK POLYESTER RESINS
Network polyester resins may be categorized into saturated and unsaturated polyesters. Unlike linear
saturated polyesters such as PET, which are made from difunctional monomers, saturated polyesters
(glyptal) are formed by the reaction of polyols such as glycerol with dibasic acids such as phthalic
anhydride.
CH2
CH
O
O
C
C
O
+
HO
CH2
CH
CH2
O
O
OH
(Str. 61)
OH
C
CH2
C
O
O
O
CH2
CH
CH2
O
glyptal
The reaction involves an initial formation of a viscous liquid that is then transferred to a mold for
hardening or network formation.
Unsaturated network polyesters may be produced by using an unsaturated dibasic acid or glycol or
both. Typically the unsaturated dibasic acid, such as maleic anhydride or fumaric acid, is copolymerized
with a saturated acid such as phthalic acid and a glycol (e.g., propylene glycol or diethylene glycol).
O
O
C
C
O
+
O
C
C
O
O
O
O
C
C
CH3
+
HO
CH2 CH
CH2
O
O
CH2
CH
CH2
O
C
(Str. 62)
OH
O
CH
CH
C
CH3
O
CH2
CH
CH2
O
CH3
The acid and glycol components are allowed to react until a low-molecular-weight (1500 to 3000) viscous
liquid (usually) is formed. This prepolymer is then dissolved in styrene monomer, which participates in
cross-linking with the double bonds in the prepolymer by addition of peroxide or hydroperoxide to form
the final network structure.
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POLYMER SCIENCE AND TECHNOLOGY
O
O
C
O
CH
CH
C
O
+
CH2
(Str. 63)
CH
O
O
C
O
CH
CH
C
O
CH2
CH
O
O
C
O
CH
CH
C
O
The saturated acid (phthalic anhydride) helps to reduce the cross-link density and, hence, the brittleness
of the cured polyester resin. Resin composition can be varied so that product properties can be tailored
to meet specific end-use requirements. For example, a resin with enhanced reactivity and improved
stiffness at high temperatures is obtained by increasing the proportion of unsaturated acid. On the other
hand, a less reactive resin with reduced stiffness is obtained with a higher proportion of the saturated acid.
Unsaturated polyester resins are light in color, easy to handle and cure rapidly without emission of
volatiles. They are dimensionally stable and have good physical and electrical properties. The major
markets for reinforced polyester resins are in marine applications, transportation, construction, and
electrical and consumer products. Fabrication of pleasure boats as well as large commercial vessels from
polyester sheet molding compounds (SMC) are typical marine applications of polyester. In transportation,
polyester resins are used in passenger-car parts and bodies and truck cabs in an attempt to reduce weight.
Polyester resins are used mainly as sheet and paneling and also as tubs, shower stalls, and pipes in the
construction industry. Consumer products from polyesters include luggage, chairs, fishing rods, and
trays. Other uses of polyester are electrical applications, appliances, business equipment, and corrosionresistant products.
X.
PROBLEMS
15.1. Explain the following observations.
a. Extremely broad MWD HDPE with polypropylene copolymer is used in the insulation of telephone
cables.
b. Polypropylene is preferred to polyethylene and polystyrene in products that must be heat or steam
sterilized.
c. Polypropylene is inert to ethanol and acetone, while solvents like benzene and carbon tetrachloride
will cause swelling.
d. To retain transparency, random copolymers of propylene and ethylene are used for films applications,
while polypropylene homopolymer is used almost exclusively for filaments.
e. Chlorinated PVC pipes are used in residential hot-water systems.
f. PVC currently has limited application in food packaging and in such applications it must meet
stringent government standards.
g. General-purpose ABS finds extensive use as refrigerator door and food liners, while fire-retardant
grades are used in appliance housing, business machines, and television cabinets and in aircraft for
interior applications.
h. Nylon is used for low- and medium-voltage 60-Hz application, for example, as secondary insulation
or jacket for primary electrical wire insulation.
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POLYMER PROPERTIES AND APPLICATIONS
463
i. Nylons 11, 12, 6,12 are preferred to nylons 6 and 6,6 for automotive wiring harness and pneumatic
tubing.
j. Principal applications of poly(phenylene sulfide) include cookware, bearings, and pump parts for
service in various corrosive environments.
k. Compared with PTFE, poly(chlorotrifluoroethylene) has a lower melting point (218°C vs. 327°C);
has inferior electrical properties, particularly for high frequency applications; and can be quenched
to quite clear sheets.
l. Butyl rubber has excellent resistance to ozone attack and is less sensitive to oxidative aging than
most other elastomers — for example, natural rubber.
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
Frados, J., Ed., The Society of the Plastics Industry, 13th ed., Society of the Plastics Industry, New York, 1977.
Ulrich, H., Introduction to Industrial Polymers, Macmillan, New York, 1982.
Waddams, A.L., Chemicals from Petroleum, Gulf Publishing Co., TX, 1980.
Chruma, J.L. and Chapman, R.D., Chem. Eng. Prog., 81(1), 49, 1985.
Fried, J.R., Plast. Eng., 38(12), 21, 1982.
Fried, J.R., Plast. Eng., 39(5), 35, 1983.
Billmeyer, F.W., Jr., Textbook of Polymer Science, 3rd ed., Interscience, New York, 1984.
Mock, J.A., Plast. Eng., 40(2), 25, 1984.
Lundberg, R.D., Ionomers, in Encyclopedia of Chemical Technology, 3rd ed., Mark, H.F., Othmer, D.F., Overberger,
C.G., and Seaborg, G.T., Eds., Interscience, New York, 1984.
1979/80 Modern Plastics Encyclopedia, McGraw-Hill, New York, 1980.
MacKnight, W.J. and Earnest, T.R., Jr., J. Polym. Sci. Macromol. Rev., 16, 41, 1981.
Fried, J.R., Plast. Eng., 39(3), 67, 1983.
Greek, B.F., Chem. Eng. News, 25, March 21, 1988.
Gogolewski, S., Colloid Polym. Sci., 267, 757, 1989.
Heineck, D.W. and Rader, C.P., Plast. Eng., 45(3), 87, 1987.
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Appendix I
Polymer Nomenclature
The IUPAC has specific guidelines for the nomenclature of polymers. However, these names are quite
frequently discarded for common names and even principal trade names. Even though there is currently
no completely systematic polymer nomenclature, there are some widely accepted guidelines that are
used to identify individual polymers.
Simple vinyl polymers are named by attaching the prefix poly to the monomer name. For example,
the polymer made from styrene becomes polystyrene. However, when the monomer name consists of
more than one word or is preceded by a letter or a number, the monomer is enclosed in parenthesis with
the prefix poly. Thus polymers derived from vinyl chloride or 4-chlorostyrene are designated poly(vinyl
chloride) and poly(4-chlorostyrene), respectively. This helps to remove any possible ambiguity.
Diene polymerization may involve either or both of the double bonds. Geometric and structural
isomers of butadiene, for example, are indicated by using appropriate prefixes — cis or trans; 1,2 or
1,4 — before poly, as in cis-1,2-poly(1,3-butadiene). Tacticity of the polymer may be indicated by using
the prefix i (isotactic), s (syndiotactic), or a (atactic) before poly, such as s-polystyrene. Copolymers are
identified by separating the monomers involved within parentheses by either alt (alternating), b (block),
g (graft), or co (random), as in poly(styrene-g-butadiene).
When side groups are attached to the main chain, some ambiguity could result from naming the
polymers. For example, poly(methylstyrene) is an appropriate designation for any of the following
structures.
CH3
CH2
CH2
C
CH2
CH
CH
CH3
n
CH3
n
(1)
(2)
n
(3)
To avoid such ambiguity, these structures are designated poly(α-methylstyrene) (1), poly(o-methylstyrene) (2), and poly(p-methylstyrene) (3), respectively.
The nomenclature of step-reaction polymers is even more complicated than that of vinyl polymers
and can be quite confusing. These polymers are usually named according to the source or initial
monomer(s) and the type of reaction involved in the synthesis. For example, nylon 6,6 (4) is usually
designated poly(hexamethylene adipamide), indicating an amidation reaction between hexamethylenediamine and adipic acid. Nylon 6 is called either poly(6-hexanoamide) or poly(ε-caprolactam). The
former name indicates the structural and derivative method while the latter, which is more commonly
used, is based on the source of the monomer.
H
N
(CH2)6
H
O
N
C
(4)
0-8493-????-?/97/$0.00+$.50
© 1997 by CRC Press LLC
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O
(CH2)4
H
C
N
n
O
(CH2)5
(5)
C
n
466
POLYMER SCIENCE AND TECHNOLOGY
Some polymers are referred to almost exclusively by their common names instead of the more
appropriate chemical names. An example is polycarbonate in place of poly(2,2-bis(4-hydroxyphenyl)
propane) (6).
CH3
O
C
O
O
CH3
C
n
(6)
The following table lists the internationally accepted abbreviations for some common commercial
polymers.
Name of Plastic
Cellulose acetate
Chlorinated poly(vinyl chloride)
Melamine–formaldehyde resins
Poly(acrylonitrile-co-butadiene)
Polyacrylonitrile
Bisphenol A polycarbonate
Polyethylene
Poly(ethylene terephthalate)
Phenol–formaldehyde resins
Poly(methyl methacrylate)
Polyoxymethylene
Copyright 2000 by CRC Press LLC
Abbrev.
Name of Plastic
Abbrev.
CA
CPVC
MF
NBR
PAN
PC
PE
PETP
PF
PMMA
POM
Polypropylene
Polystyrene
Polytetrafluoroethylene
Polyurethane
Poly(vinyl acetate)
Poly(vinyl alcohol)
Poly(vinyl butyral)
Poly(vinyl chloride)
Poly(vinylidene fluoride)
Poly(vinyl pyrrolidone)
Urea–formaldehyde resins
PP
PS
PTFE
PUR
PVAC
PVAL
PVB
PVC
PVDC
PVP
UF
POLYMER NOMENCLATURE
Appendix II
Answers to selected problems
Chapter 1
1.5 (a) 1.13 × 105; (b) 1.92 × 105; (c) 1.18 × 105; (d) 2.54 × 105
Chapter 3
3.3 Mn = 3770 g/mol; M2 = 31,000 g/mol; Mw /Mn = 8.2
3.4 Mn = 2.35 × 105 g/mol, Mw = 7.36 × 105
3.10 8.1
Chapter 4
4.2
4.4
4.5
4.6
4.7
VB = 32.4%
Xn = 50
Tm = 286.5°C
Tm = 194°C
f = 0.028
Chapter 5
5.10 (a) 59.26 phr; (b) 296.30 phr
Chapter 6
6.1
6.2
6.3
6.4
6.5
6.6
(a) Mn = 5650 g/gmol; (b) Xn = 30.45; (c) Mn = 11,300 g/gmol; (d) Mn = 7533 g/gmol
(a) Mn = 6000 g/gmol; Mw = 8667 g/gmol; (b) melt viscosity will increase
(a) Mn = 2504 g/gmol; (b) Mn = 4407 g/gmol
(1) 2; 100; 00; (b) Xn = 199; (c) Xn = 49
(a) Mn = 9600; (b) Mn = 19,104; (c) 9.80 × 10–3; (d) 2.94 × 10–6
For n = 1 p = 0.1, Wx = 0.98
P = 0.9, Wx = 0.01
For n = 100 p = 0.1, Wx = 8.1 × 10–98
P = 0.9, Wx = 2.95 × 10–5
6.7 Nylon 6, Mn = 3260
Nylon 12, Mn = 3240
6.8 Fraction of monomers = 4.0 × 103
Chapter 7
(b) Ri = 7.5 × 10–11 mol/ml, Xn = 4.01 × 103; (c) 94.3%
(a) Rp = 0.715[I]1/2[M]; (b) Rp = 0.044 mol/l.s.; (c) 15 ls
(a) CM = 0.6 × 10–4; (b) Xn = 602; (c) Xn = 833; (d) V = 415; (e) G = 0.055; (f) f = 0.61
Cyclohexane Mn = 3.69 × 105
Carbon tetrachloride Mn = 2.87 × 103
7.6 Styrene [S]/[M] = 19.35
Methyl methacrylate [S]/[m] = 11.54
Vinyl Aceptate [S][M] = 0.07
7.10 (a) Mn = 1.04 × 106; (b) Mn = 2.08 × 106; (c) Mn = 1.49 × 105
7.11 (a) 250 mn; (b) E = 7.3 Kcal/mol
7.2
7.3
7.4
7.5
Chapter 9
9.2 EL = 2.88 × 106 psi
Copyright 2000 by CRC Press LLC
467
468
POLYMER SCIENCE AND TECHNOLOGY
Chapter 10
10.2
10.3
10.4
10.5
10.6
10.8
10.9
10.10
10.11
10.12
10.16
t = 11.1 h
t = 5.96 × 10t s
[M]/[Mo] = 94.9%
∆T = 617°C
(Xn)b/(Xn)s = 4.75
Ratio of total surface area of micelles to droplets = 20 × 107
(a) t = 0.40 h; (b) D = 4.44 × 10–5 cm
Tc = 30.1°C
(a) τ = 2.53 h; (b) flow rate = 19.73 m3/h
(a) p = 0.632; (b) L = 100 m
Feed temperature = 50°F
Chapter 11
11.1
11.3
11.4
11.5
Power Requirement = 7–18 hp
(a) Polystyrene Q = 591/b/h; (b) Polyethylene Q = 259/b/h; (c) Nylon 6,6 Q = 329/b/h
(a) 176 hp; (b) 233 hp
(a) 1.3%; (b) 2.1%; (c) 3.6%
Chapter 12
12.1 Polymer Most Suitable Solvent
Natural Rubber Dichlorobenzene
Polyacrylonitrite Nitromethane
12.4 (a) 6.9; (b) 1.86; (c) 6.04; (d) 3.47 × 104 A°
12.6 P1 = 92 mmHg
12.7 M = 1.58 × 106
12.8 (r2of) 1/2 = 302 A°
12.9 (a) 3.98 × 104 A°; (b) r = 76 A°; (c) (r2of) 1/2 = 750 A°
12.11 k = 0.561; Mn = 2.22 × 105 g/mol
12.12 (a) Solution A: Mn = 1.89 × 105 g/mol
Solution B: Mn = 1.01 × 105 g/mol
(b) Mn = 1.14 × 105 g/mol
(c) Mw /Mn = 2.16
Chapter 13
13.1
13.4
13.5
13.6
79.78 × 104 J/m3
7.07 × 105 N/m2
2 × 102 N
67.55 × 106 N/m2
Chapter 14
14.2
14.3
14.4
14.5
14.6
14.7
14.8
78.04 × 105 N/m2
0.517
0.701
83°C
2.5 × 102 s
194°C
1.29 × 10–3
Copyright 2000 by CRC Press LLC
469
POLYMER NOMENCLATURE
Appendix III
Some Useful Conversion Factors
To Convert From
atmosphere (760mm Hg)
Btu
calorie
centipoise
foot
ft-lbf
gallon (U.S. liquid)
horsepower
inch
inch of mercury (60°F)
inch of water (60°F)
kilogram-force (Kgf)
micron
pound-force (lbf)
lbf /in.2 (psi)
watt-hour
yard
To
Multiply By
pascal (Pa)
joule (J)
joule (J)
pascal-second (Pa·s)
meter (m)
joule (J)
cubic meter (m3)
watt (W)
meter (m)
pascal (Pa)
pascal (Pa)
newton (N)
meter (m)
newton (N)
pascal (Pa)
joule (J)
meter (m)
1.013 × 105
1.055 × 103
4.187
1.00 × 10–3
3.048 × 10–1
1.356
3.785 × 10–3
7.460 × 102
2.540 × 10–2
3.377 × 103
2.488 × 102
9.807
1.000 × 10–6
4.448
6.895 × 103
3.600 × 103
9.144 × 10–1
Values of Some Useful Physical Constants
cgs
Avogadro’s number, No.
Velocity of light, c
Boltzmann’s constant, K
Gas constant, R
Copyright 2000 by CRC Press LLC
6.02
3.00
1.38
8.31
×
×
×
×
1023 molecules/mol
1010 cm/s
10–16 erg/K
107 erg/g mol·K (1.98 cal/mol·K)
SI
6.02
3.00
1.38
8.31
×
×
×
×
1023 molecules/mol
108 m/s
10–23 J/K
103 J/kg mol·K
SOLUTIONS TO CHAPTER 1 PROBLEMS
1.1.
Monomer
Repeating Unit
a. CH2
CH
COOH
Polymer Name
Poly(acrylic acid)
CH2CH
COOH
b. CH2
CH3
O
C
C
CH3
O
CH3
CH2
Poly(methyl methacrylate)
C
C
O
O
CH3
O
c. CH
2
CH
O
C
CH2
CH3
CH
Poly(vinyl acetate)
O
C
O
CH3
d. CH2
CH
CH3
CH2
CH
Polypropylene
CH3
e. CH
2
CH
CN
CH2
CH
Polyacrylonitrile
CN
O
1.2.
a.
H2N
(CH2)5
NH2 + Cl
pentamethylenediamine
H
N
(CH2)6
C
O
(CH2)5
C
Cl
pimeloylchloride
H
O
N
C
O
(CH2)5
C
poly(pentamethylene pimelamide); nylon 5,7
0-8493-????-?/97/$0.00+$.50
© 1997
by CRC
Press
Copyright 2000
by CRC
Press
LLCLLC
1
2
POLYMER SCIENCE AND TECHNOLOGY
b.
HOOC
COOH + HO
terephthalic acid
(CH2)10
OH
1,10-decanediol
O
O
C
C
O(CH2)10
O
poly(decamethylene terephthalate)
O
c.
HO
CH2
CH
CH2
OH
+
O
C
C
O
OH
glycerol
O
CH2
CH
phthalic anhydride
CH2
O
O
O
C
C
CH2CH2
OH
O
glyptal
d.
CH3
NCO
+
HO
NCO
toluenediisocyanate
CH3
H
O
N
C
ethylene glycol
O
O
NH
Copyright 2000 by CRC Press LLC
C
polyurethane
CH2CH2
O
3
1.3.
Monomer
a. CH
2
Repeating Unit
CH
CH2
C
b. CH2
O
O
O
CH2
CH2
CH3
CH3
CH3
CH3
C
CH2
CH3
c. CH
2
CH2
CH3
Cl
Cl
C
CH2
CH2
O
C
NH
C
H
(CH2)5
N
Polycaprolactam (nylon 6)
(CH2)5
O
Polycaprolactone
O
CH2
CH2
O
CH2CH2
f. H2C
C
O
C
O
H2C
CH2
CH2
CH3
Si
CH3
Cl
CH3
Copyright 2000 by CRC Press LLC
Poly(vinylidene chloride)
C
Cl
e. CH
2
h.
Polyisobutylene
C
Cl
g. Cl
Poly(α-methyl styrene)
C
CH3
CH2
O
CH3
C
d. CH
2
Poly(ethyl acrylate)
CH
C
Polymer
O
Si
CH2
CH2
CH2
Poly(dimethyl siloxane)
(CH2)4
Poly(tetramethylene oxide)
CH3
O
CH2
O
4
POLYMER SCIENCE AND TECHNOLOGY
1.4.
Polymer
Yes
Monomer A
Monomer B
O
a. R
HO
R´
C
No
Linear
Branched/
Cross-linked
OH
X
OH
OH
b. HOOC
c. HO
R
R
HO
COOH
OH
CH
R´
R´
OH
N
C
X
X
O
CH2
d.
X
CH2
CH
e. HO
(CH2)5 COOH
f. H2N
R
X
NH2
HOOC
R´
X
COOH
NH2
g.
CH
O
CH
C
C
CH
CH2
CH
CH2
X
O
O
h. H2N
R
NH2
OCN
R´
X
NCO
CH3
i.
CH2
X
C
CH3
j. CH2O
H2N CH2
O
k. CH2
O
X
CH2
O
H
1.5.
a.
N
O
(CH2)5
C
1000
Molecular weight of repeat unit = 15 + (14 × 5) + 28 = 113
Mwpolymer = (MWrepeat unit)(DP) = 113 × 1000
= 1.13 × 105
Copyright 2000 by CRC Press LLC
X
CH2NH2
CH2
5
b.
O
CH2
CH2
O
O
O
C
C
1000
Molecular weight of repeating unit = 16 + (14 × 2) + 16 + 28 + (6 × 12) + 4 + 28
Mwpolymer = (MWrepeat unit)(DP) = 192 × 1000
= 1.92 × 105
c.
CH2
CH
CH3
1000
Molecular weight of repeat unit = 14 + 13 + (6 × 12) + 4 + 15 = 118
Mwpolymer = 118 × 1000 = 1.18 × 105
CH3
d.
O
O
C
O
C
CH3
1000
Molecular weight of repeat unit = 16 + (12 × 6) + 4 + 12 + (2 × 15) + (12 × 6) + 4 + 16 + 28
= 254
Mwpolymer = MWrepeating unit = 254 × 1000
= 2.54 × 105
O
1.6.
a.
O
(CH2)2
O
b.
O
(CH2)2
O
c.
O
(CH2)6
O
d.
O
(CH2)4
O
C
O
(CH2)2
O
C
O
C
(CH2)8
O
O
C
C
O
O
C
C
C
1.7.
O
CH
CH
C
O
hydrolysis
C
O
C
CH
CH
maleic acid
O
maleic anhydride
Copyright 2000 by CRC Press LLC
HO
O
C
OH
6
POLYMER SCIENCE AND TECHNOLOGY
O
HO
C
O
CH
C
CH
OH
+ HO
CH2
CH2
CH2
CH2
OH
step-reaction polymerization
O
C
O
CH
C
CH
O
CH
(CH2)4
O
CH2
free-radical polymerization
O
C
O
CH2
CH
C
O
(CH2)4
O
CH2
CH
This is a polyester with polystyrene grafted onto the backbone.
O
HO
C
O
(CH2)4
O
C
C
OH
+
HO
(CH2)4
OH
O
(CH2)4
C
O
(CH2)4
O
This is a polyester without a possibility of reaction with styrene since there are no residual double
bonds in the main chain.
1.8.
Copyright 2000 by CRC Press LLC
7
This residual double bond in the main chain permits vulcanization.
1.9.
To be a good adhesive, the reactants must be capable of forming a cross-linked structure. In this reaction,
formaldehyde behaves as a glycol HO–CH2–OH (its hydrated form). It is therefore bifunctional. To
form a cross-linked structure with formaldehyde, therefore, the phenolic compound must be trifunctional.
From elementary organic chemistry, only compound C is trifunctional and should therefore be chosen.
OH
R
R
1.10.
Copyright 2000 by CRC Press LLC
n
Name of Amine
Functionality
1
2
3
4
Diethylenetriamine
Triethylenetetramine
Tetraethylenepentamine
Pentaethylenehexamine
5
6
7
8
8
POLYMER SCIENCE AND TECHNOLOGY
SOLUTIONS TO CHAPTER 2 PROBLEMS
2.1. The ease of polymerization of a vinyl monomer depends on the reactivities of the monomer and the
attacking radical. Substituents that stabilize the product radical tend also to stabilize the monomer but
to a much smaller extent. Resonance stabilization depresses the reactivity of radicals. In contrast to
vinyl acetate, butadiene has conjugated double bonds. Using 1,2 polymerization of butadiene as an
example, it is apparent that the product radical from butadiene is much more resonance stabilized than
that from vinyl acetate. In fact, the monomer–radical propagation rate constants at 60°C are 100 and
2300 l/mol-s for butadiene and vinyl acetate, respectively.
a. CH2
CH
CH2
CH
CH
O
CH2
C
O
CH3
vinyl acetate
butadiene
b.
2.2.
The higher the initiator concentration, the higher the number of chain radicals initiated. Therefore,
for a fixed monomer concentration, the average number of monomers consumed by each chain
radical will be reduced as the initiator concentration increases.
Phthalic anhydride is a saturated acid. Its incorporation into the resin mixture serves to decrease the
overall cross-link density and, consequently, resin brittleness.
2.3. The melting point of the polymer decreases with increasing flexibility, i.e., increasing value of n.
H2N (CH2)2 NH2
ethylenediamine
H2N (CH2)10 NH2
dexcamethylenediamine
O
O
C
C
N
N
C
O
C
(CH2)n
x
O
aromatic polyimide
Poly(decamethylene pyromellitimide), n = 10, is more likely to form a useful polymer than poly(ethylene
pyromellitimide), n = 2. In fact, diamines whose carbon chain contains less than seven carbon atoms
form polymers that degrade at or below their melting points.
2.4.
The polymers from the three compounds will have ladder-type structures. However, the polymers from
compounds B and C will have at least one and two skeletal bond(s) per repeating unit while a doublecyclization polymer will result from compound A. The presence of one or two consecutive single bonds
between aromatic units will enhance polymer susceptibility to thermal degradation. In principle, therefore, under identical conditions the thermal stability of the resulting polymers will be A more than B
more than C.
Copyright 2000 by CRC Press LLC
9
2.5. The presence of substituent alkyl groups on the phenolic ring renders the resulting phenolic resins oilsoluble.
OH
CH3
C
OH
CH3
CH3
p-tertiary butyl phenol
2.6.
CH3
C
OH
CH3
C2H5
p-tertiary amyl phenol
CH3
n
C
CH3
C5H11
p-tertiary octyl phenol
In free-radical polymerization, high molecular weight is formed at a very early stage of polymerization
(line A). In condensation polymerization, high-molecular-weight polymer is formed only toward the
end of the reaction, i.e., conversion approaches 100% (line B). In living polymerization, the molecular
weight is directly proportional to conversion.
Copyright 2000 by CRC Press LLC
10
POLYMER SCIENCE AND TECHNOLOGY
SOLUTIONS TO CHAPTER 3 PROBLEMS
3.1a.
CH3
CH2
C
CH3
polyisobutylene
(PIB)
CH2
CH2
C
C
CH3
H
poly(1,4-cis-polyisoprene)
(natural rubber)
PIB is a saturated polymer. Natural rubber has a residual double bond on its main chain and this
is susceptible to oxidative attack.
b.
CH2
CH2
CH2
polyethylene (nonpolar)
CH
poly(p-chlorostyrene) (polar)
Cl
c.
Poly(p-chlorostyrene) is polar and therefore is not suitable as an electrical insulator. Polyethylene,
on the other hand, is nonpolar and will not conduct electricity.
PMMA is amorphous; there are therefore no regions of differing indices of refraction in the polymer.
Hence, it is transparent. At room temperature, polyethylene is semicrystalline, having both crystalline and amorphous components that differ in their refractive indexes. Consequently, PE is translucent. At 135°C, PMMA is still amorphous, while polyethylene also becomes amorphous since
its melting point is 135°C. Both polymers are then transparent.
d.
CH2
CH2
polyethylene
CH3
CH2
C
polyisobutylene
CH3
In contrast to polyethylene, it is impossible sterically to accommodate the two methyl groups in
polyisobutylene in the planar zigzag conformation. However, a helical conformation can accommodate the pendant groups.
e.
O
C
H
(CH2)5
O
C
Copyright 2000 by CRC Press LLC
N
nylon 6
H
(CH2)11
N
nylon 12
11
f.
g.
h.
i.
For the same degree of polymerization, the density of the hydrophilic amide linkages –NHCO– is
higher, while the length of the hydrophobic methylene (–CH2–) group is smaller in nylon 6 than
in nylon 12.
Small amounts of chlorine produce irregularities in the polymer structure. This reduces crystallinity
and hence lowers the softening point. Large amounts of chlorine in the structure lead to resumed
crystallinity in addition to higher polarity. Thus the softening point increases.
The addition of glass fibers increases the number of nucleation sites in semicrystalline polymers.
This produces an increase in the degree of crystallinity and, consequently, an enhanced heat
distortion temperature. By definition, amorphous polymers are noncrystalline and, as such, this
mechanism is inoperable.
The high cohesive energy density of polyethylene makes it highly crystalline and insensitive to solvents
except at temperatures close to its melting point (about 135°C). Thus, even though gasoline and PE
are both hydrocarbons, gasoline will not dissolve PE when used in an automobile gasoline tank.
Teflon is nonpolar due to high degree of symmetry of the starting monomers, has an extremely
high CED because the small size of the fluorine atom permits close packing of molecular chains,
and is not attacked by chemicals.
CH2
j.
polyethylene (nonpolar)
CH2
O
H
C
(CH2)2
N
nylon 6 (polar)
The predominant intermolecular bonds in nylon 6 are hydrogen bonds resulting from the highly
polar –NHCO– amide linkages. On the other hand, polyethylene is held together by van der Waals
forces, which are much weaker than the polar hydrogen bonds in nylon 6.
3.2.
Styrene is soluble in CH3OH but not in H2O. Precipitation with H2O will consequently bring down both
the unreacted styrene monomer and polystyrene. No, attempting to distill off the monomer would
probably initiate thermal polymerization of styrene.
3.3.
Mn =
∑W
∑W M
i
i
=
i
200
100
50
50
+
+
2 × 103 2 × 10 4 1 × 10 5
= 3770 g mol
Mw =
∑WM
i
i
= 100 200 × 2 × 10 3 + 50 200 × 2 × 10 4 + 50 200 × 1 × 10 5
= 31, 000 g mol
Polydispersity =
3.4.
Mn =
∑W
∑W M
i
i
Copyright 2000 by CRC Press LLC
Mw 31, 000
=
= 8.2
3770
Mn
i
12
POLYMER SCIENCE AND TECHNOLOGY
Mn =
200 + 200 + 100
200 1.2 × 10 5 + 100 5.6 × 10 5 + 100 10.0 × 10 5
= 2.35 × 10 5 g mol
M w = ∑ Wi M i =
200
100
200
× 4.5 × 10 5 +
× 8.9 × 10 5 +
× 10 5
500
500
500
= 7.36 × 10 5 g mol
3.5.
Polymer
Xn
Assignment
P
V
1500
2000
Poly(vinyl chloride)
Poly(vinylidene chloride)
Polymer
Repeating Unit
Poly(vinyl chloride)
–CH2–CH–
|
CL
–CL
|
–CH2–C–
|
CL
Poly(vinylidene chloride)
Comment
Polar
Nonpolar
PVC is a polar polymer, whereas poly(vinylidene chloride), due to symmetry, is nonpolar. Its cohesive
energy is derived essentially from van der Waals forces. This means that it will require a higher number
of monomers strung together to develop good physical properties.
3.6.
As a result of the reactions that occur during free-radical polymerization, the reacting monomer(s) may
be arranged in different ways on the polymer chain. Consequently, most free-radical polymerizations
yield atactic polymers, which are noncrystalline. However, in cases where the pendant group is small
enough to fit into a crystalline lattice, crystalline polymers are formed — for example, poly(vinyl
alcohol). Besides, if the monomer is symmetrical, free-radical polymerization may also yield crystalline
polymers, e.g., polyethylene and polytetrafluoroethylene.
CED =
∆H V − RT
V1
RT = (1.98 cal mol K ) (273 + 25 K)
3.7.
= 590 cal mol
(
δ = CED cal cm 3
Solvent
Copyright 2000 by CRC Press LLC
2,2,4-Trimethylpentane
n-Hexane
CCL4
CHCl3
Dioxane
CH2Cl2
CHBr3
Acetonitrile
)
12
CED (cal/cm3)
δ (cal/cm3)1/2
47.0
52.8
73.9
85.8
94.8
99.4
111.9
139.6
6.90
7.3
8.6
9.3
9.7
10.0
10.6
11.8
13
From graph, δ2 (polymer) = 9.3
CED = δ 2 = 86.5 cal cm 3
∆H m = 1 2 (1 − 1 2) (11.8 − 9.3)
2
= 1.56 cal cm 3
3.8.
a.
b.
The crystalline form of a polymer is always denser than the amorphous form because of more
efficient molecular packing. In a semicrystalline polymer this difference in densities between the
amorphous and crystalline regions of the polymer determines its transparency since the index of
refraction is proportional to the density. Similarly, polymer rigidity depends on the degree of
crystallinity. Polyethylene and polypropylene produced by stereospecific catalysts are semicrystalline. The polymers are translucent because of the difference in the refractive indices between the
amorphous and crystalline components. With copolymerization, however, there is a mutual destruction
of crystallinity. The copolymer is amorphous, soft, and transparent since there no longer exist
regions of differing refractive indices.
In this case a block copolymer is produced. The sequences of ethylene and polypropylene units
are long enough to crystallize independently. The individual properties of the separate polymers
are retained by the block copolymer.
3.9.
Absorption
H2 O
Hexane
Responsible
Structure
Comments
H O
| ||
–N–C (amide)
–CH2– (methylene)
As n increases, water absorption becomes low because the
hydrocarbon nature of the polymer predominates. Consequently,
hexane absorption increases with n. On the other hand, as n
decreases, ability for hydrogen bonding through the amide linkages
is enhanced. Water absorption increases while hexane absorption
decreases.
3.10. The copolymer may be regarded as having a structure in which glassy polystyrene acts as crosslinks
for the rubbery polybutadiene. For such a lightly cross-linked polymer, solubility (swelling) is highest
when the solubility parameter of the polymer δ2 is equal to that of the solvent. The solubility parameter
of a 1:1 mixture of pentane and ethyl acetate is given by:
δ = 1 2 (7.1) + 1 2 (9.1) = 1 2 (7.1 + 9.1)
= 8.1
3.11.
Material (urea derivative)
(1) Hexamethylenediamine
(2) Tris(2-aminoethylamine)
(3) Triethyleneoxide diamine
(4) Poly(oxypropylene triamine)
Copyright 2000 by CRC Press LLC
Comments
Flexible but symmetrical molecules; –NH–CO–NH– group enhances
CED and as such crystallizes readily and with high melting point.
Less flexible than (1), and branched but trifunctional. Crystallization
tendency reduced, CED also reduced, hence relatively lower melting
point.
Flexible ether, difunctional. Introduction of oxygen atom into the main
chain reduces rotational energy barrier. Enhanced flexibility
considerably reduces crystallization tendency and also crystalline
melting point.
Polymeric and branched nature makes crystallization impossible.
Reduced CED and hence a liquid at room temperature.
14
POLYMER SCIENCE AND TECHNOLOGY
3.12. Proper molecular alignment is a requirement for crystallinity. A dilute solution allows sufficient molecular translation to meet this requirement. In contrast, given the extensive molecular entanglements in a
polymer melt, enough molecular alignment is impossible, and hence crystal perfection is smaller than
that from a dilute solution. However, at high temperature sufficient molecular mobility occurs for proper
molecular alignment, and consequently, enhanced crystallinity is obtained.
Copyright 2000 by CRC Press LLC
15
SOLUTIONS TO CHAPTER 4 PROBLEMS
4.1.
Increasing order of Tm: e < a < b < d < c.
Material
Comments
H
H
c
With linkages
N C N , forms strong H-bonds; phenyl rings
stiff and rigid. Both increase Tm
d
b
Urea linkages; high density of hydrogen bonding
Amide linkages; same density of hydrogen bonding as d but urea linkages generally stronger
than amide linkages
Amide linkages but relatively smaller density of hydrogen bonding than b due to longer
distances between amides; (CH2)8 vs. (CH2)4
Ester linkages enhance molecular rotation and hence reduce Tm
a
e
4.2.
O
are
Since PVC and dibutyl sebacate have about the same value of solubility parameters, they are compatible
and hence the following expression is applicable:
Tg = VA TgA + VBTgB
VA and VB are volume fractions of polymer and plasticizer, respectively.
25 = (1 − VB ) 85 + VB (−100)
VB =
60
= 32.4%
185
4.3.
Polymer
Structure
O
a. Poly(ethylene adipate)
b. Poly(ethylene terephthalate)
O
O
(CH2)2
(CH2)2
O
O
C
O
(CH2)4
C
n
O
O
C
C
n
O
c. Poly(hexamethylene adipate)
O
(CH2)6
H
d. Poly(ethylene adipamide)
Copyright 2000 by CRC Press LLC
N
(CH2)2
O
C
H
O
N
C
O
(CH2)4
C
n
O
(CH2)4
C
n
16
POLYMER SCIENCE AND TECHNOLOGY
Order of decreasing melting point: b > d > a > c.
4.4.
S
S
S
Polymer
Comments
b
d
a
c
Presence of phenyl rings stiffens backbone and increases Tm
Amide linkages form hydrogen bonds
Ester linkages more flexible than amide linkages in (d)
Ester linkages; and more flexible than (a); (CH2)6 vs. (CH2)2
S
S
D
S
S
S
S
D
S
S
S
S
S
S = styrene residue, D = divinyl benzene residue
Tgc − Tg =
3.9 × 10 4
Mc
Tgc = Tg of cross-linked polymer
Tg = Tg of uncross-linked polymer
7.5 =
3.9 × 10 4
Mc
M c = 5200
Mc = Xn Mo
M o = molecular weight of styrene (CH 2 − CH −) = 104
Xn =
4.5.
M c 5200
=
= 50
Mo
104
Tg
Tm
=
2
for unsymmetrical polymers
3
Tg = 100°C = 273 + 100 = 373 K
Tm =
4.6.
3
× 373 = 559.5 K = 286.5°C
2
R
1
1
ln x
=
−
Tm
Tm° ∆H m
Tm = 250 + 273 = 423 K
1.98 (cal mol)
1
1
ln 0.75
=
−
Tm 423 2500 cal mol
= 0.0021
Tm = 467 K = 194°C
Copyright 2000 by CRC Press LLC
17
4.7.
Polymer
Structure
CH3
Poly(2-chloroethyl methacrylate)
CH2
C
C
O
CH2CH2Cl
I
CH3
Poly(n-propyl methacrylate)
CH2
C
C
O
O
CH2CH2CH3
II
CH3
Poly(n-butyl methacrylate)
CH2
C
C
O
O
CH2CH2CH2CH3
III
CH3
Poly(2-methoxyethyl methacrylate)
CH2
C
C
O
O
CH2CH2OCH3
IV
Poly(2-chloroethyl methacrylate) is more polar than poly(n-propyl methacrylate) due to the chlorine
atom on the side of the chain. Similarly, poly(2-methoxyethyl methacrylate) is more polar than poly(nbutyl methacrylate). The glass transition temperatures of the more polar polymers are higher.
4.8. f = f + α T − T
g
g
= 0.025 + 60.6 × 10 −6 (150 − 100) = 0.028
Copyright 2000 by CRC Press LLC
18
POLYMER SCIENCE AND TECHNOLOGY
4.9.
Polymer 1
Polymer 2
Comments on Relative Tm
Polyethylene
[CH2 CH2]n
Random copolymer of ethylene and
propylene
Tm (polymer 1) > Tm (polymer 2).
Polymer 1 is highly crystalline while
copolymerization disturbs molecular
order and hence reduces crystallinity
of copolymer.
O
C
Poly(vinyl chloride) (PVC)
CH2
CH
(CH2)5
N
n
Polytetrafluoroethylene (Teflon)
CF2
n
H
CF2
n
Cl
Nylon 6
Nylon 11
O
C
Copyright 2000 by CRC Press LLC
H
(CH2)5
O
N
C
n
H
(CH2)10
N
n
Tm (Teflon) > Tm (PVC) because of
small size of fluorine atom; Teflon is
highly symmetrical hence it has high
CED.
Tm (1) > Tm (2). Density of amide
linkages –(NHCO–) is higher in nylon
6 than nylon 11. Consequently,
nylon 6 has higher CED resulting
from H-bonding. Nylon 11 is more
flexible than nylon 6 (longer –CH2–
sequences).
19
SOLUTIONS TO CHAPTER 5 PROBLEMS
5.1.
The compatibilizing agent prevents the migration of the blended homopolymers. This restriction in
mobility minimizes the volume occupied by each polymer in its environment within the material.
Consequently, molecules of each polymer in the blend form higher coils compared with the respective
homopolymers. In addition, to improve chemical and solvent resistance, a major consequence of such
reduction in the number of entanglements is improved flow properties, i.e., a higher melt index. Recall
that melt index is defined as the weight of polymer flowing through a given orifice under certain
prescribed standard conditions.
5.2.
In order to obtain adhesives of high solids content, the dextrin must be highly soluble in water. The
starch degrative product with a lower degree of polymerization will be expected to be more soluble.
Given the manufacturing conditions of white and yellow dextrins, canary dextrins will be expected to
have a lower average degree of polymerization and, consequently, are more suitable as envelope seal
adhesives.
5.3.
a. Structure of nylon 6:
Presence of amide linkages favors hydrogen bonding and, consequently, water absorption. A
decrease in the intensity of H-bonding and presence of hydrophobic groups should reduce water
absorption. Copolymerize, for example, H2N(CH2)10-NH2.
b. A polymer that possesses the ability to flow is generally tough and capable of blunting crack
propagation. The flow properties of PE can be increased by decreasing its crystallinity or density by:
1. Copolymerization of butene, hexene, or octene
2. Copolymerization of small quantities of acrylic acid
3. Copolymerization of acrylate (CH2› CH–CO–O–R) where R is an alkyl group with molecular
weight higher than CH3
c. Copolymerization of rubber (butadiene) to generate a microstructure of dispersed rubber phase
within a glassy styrene matrix. Rubbery phase helps to prevent propagation of microcracks and
thus increase impact resistance.
5.4.
Structure of copolymer
Recall that melt index, by definition, is amount of polymer flowing through a given orifice under some
specified conditions. Introduction of the polar acrylic acid unit into the polymer structure increases its
polarity while at the same time disrupting the interchain bonding and crystalline nature of PE. Initially,
the disruption of the interchain interaction of PE predominates. Consequently, flow is enhanced and
melt index increases. Further increase in acrylic acid content results in the predominance of chain
polarity and hence intermolecular attraction. This leads to a decrease in the flow properties.
As the polarity of the copolymer increases with the increasing content of acrylic acid, the bonding with
ionic groups on A1 increases. The peel strength therefore increases correspondingly.
5.5.
The structure of nitrile rubber:
CH2
CH
x
CN
Copyright 2000 by CRC Press LLC
CH2
CH
y
CH
CH2
CH2
CH
CH
CH2
z
20
POLYMER SCIENCE AND TECHNOLOGY
Nitrile rubber is a copolymer of acrylonitrile and butadiene. The poor resistance to oil is due to the
butadiene component. Acrylonitrile is highly polar, and its introduction into the structure therefore
enhances the oil resistance of the copolymer, the extent of which depends on its content in the copolymer.
5.6.
Copolymerization of the following monomers with the starting monomer(s) of the given homopolymer
should, in principle, bring about the desired changes. The extent to which these changes are realized
will, however, depend on the composition of the comonomers.
O
O
C
(ia)
HO
C
(ib)
CH2
CH
OH Introduces rigid phenyl rings into polymer backbone
Polarity of monomer (due to presence of CN group) enhances
intermolecular attraction and hence reduces the possibility of
CN chemical attack
O
O
(ic)
HO
C
(id)
CH2
CH
(CH2)4
C
OH Presence of (CH2)4 should enhance overall flexibility
Increases polymer polarity
COOH
(ie)
O
O
C
C
O
Introduces aromatic rings with imide or
imidazole units into polymer structure
O
C
C
O
O
or
H2N
NH2
H2N
NH2
CH
CH2
Affects cross-linking due to increased functionality
(iia)
CH
(iib) HO
CH2
CH2
CH
CH2
OH
Trififunctionality of monomer results in cross-linking
OH
5.7.
The reactivity ratios of the components are
Maleic anhydride (1)
Methyl methacrylate (2)
Vinyl acetate (2)
r1
0.02
0.003
r2
6.7
0.055
r1 r2
0.134
0.00017
Based on these reactivity ratios, the tendency toward alternation is much greater with vinyl acetate as
the curing monomer. Thus the polymer resulting from cure of the polyester with vinyl acetate will
consist of short but numerous cross-links, while that cured with methyl methacrylate will consist of
Copyright 2000 by CRC Press LLC
21
long but relatively fewer cross-links. Good creep resistance demands high cross-link density; i.e., cure
with vinyl acetate will be preferable for this application.
5.8.
The brittleness of films from cured UF resins may be attributed to the strong hydrogen bonding of the
urea (–NH–CO–NH) interunit linkages and the stiff methylene (–CH2–) units. Any structure that will
increase the flexibility of UF polymer chain will therefore reduce the brittleness of the cured UF resin.
The phenyl rings will enhance stiffness, while long sequences of (CH2)2 will enhance flexibility.
Consequently, the brittleness of cured UF films will be reduced in the order: b < c < d < a.
5.9.
The introduction of flexibilizing molecular units into the polymide structure should produce a useful
adhesive. Therefore, compound B would be preferable to compound A. The presence of the – –O–
reduces molecular rotational energy and thus enhances flexibility. The resulting polymer structure has
both imide and benzimidazole units, which are characterized by high aromatic and low hydrogen
contents. The structure is very rigid and therefore should possess high temperature resistance.
5.10. Vulcanization reaction:
Basis: 100 g rubber. From stoichiometry therefore:
100 g rubber ×
n g mol S
1 g mol rubber
32 g S
×
×
2 g mol rubber 1 g mol S
54 g rubber
=
a.
b.
3200 n
gS
108
For disulfide cross-links, n = 2. Amount of sulfur = 59.26 phr.
For decasulfide cross-links, n = 10. Amount of sulfur = 296.30 phr.
5.11. UF resin stability to hydrolysis will be expected to increase in the order: monomer 3 > monomer 1 >
monomer 2. UF resins as result of their urea linkages [HN –CO–NH] are capable of absorbing water
through hydrogen bonds. Inclusion of these monomers into the resin structure will break the frequency
of these linkages and therefore reduce the possibility of water absorption and, consequently, hydrolysis.
The 12 and 6 methylene groups in monomers 3 and 1, respectively, will be expected to increase the
hydrophobicity of the resins, with monomer 3 being more effective than monomer 1. On the other hand,
monomer 2, while reducing overall water uptake, is still capable of encouraging water absorption because
of the oxygen atoms.
5.12.
CH2
CH2
Polyethylene
n
CH2
CH2
O
Poly(ethylene oxide)
n
The presence of oxygen atoms in poly(ethylene oxide) would be expected to enhance its flexibility
relative to polyethylene. Stiff main chains apparently permit more independent movement of side chains.
Therefore, the number of methylene spacer groups required to eliminate interference on the reactivity
of functional groups from the polymer backbone is less for the less flexible polyethylene chains than
Copyright 2000 by CRC Press LLC
polyoxyethylene.
22
POLYMER SCIENCE AND TECHNOLOGY
SOLUTIONS TO CHAPTER 6 PROBLEMS
O
6.1.
n HO
C
O
(CH2)4
C
adipic acid
O
C
(CH2)4
OH + n H2N
(CH2)6
NH2
hexamethylenediamine
O
H
C
N
H
(CH2)6
N
+ 2n H2O
n
nylon 6,6
a.
b.
Xn =
1
226
; M0 =
= 113
1− p
2
Mn =
Mo
113
113
=
=
= 5650
1 − p 1 − 0.98 0.02
Xn =
1+ r
2 r (1 − p) + 1 − r
r=
Xn =
100
= 0.98
102
1 + 0.98
1.98
=
= 30.45
2 (0.98) (1 − 0.98) + 1 − 0.98 0.0592
M n = M o X n = 3779
c.
M n = 113 × 100 = 11, 300; X n =
d.
Mn =
1
= 100
1 − 0.99
∑ NiMi
∑ Ni
Consider 1 mol of usual product, i.e., ΣNi = 1
NA + NN = 1, NA = mol of afternoon shift
NN = mol of night shift
NA (3779) + (1 – NA) (11,300) = 5650
NA (3779 – 11,300) = 560 – 11,300
N
A
N
B
=
= 1 − 0.75 = 0.25
Mn =
6.2.
5650
= 0.75
7521
∑ N M 2 0.75 (3379)2 + 0.25 (11, 300)2
i i =
= 7533
∑N M
0.75 (3779) + 0.25 (11, 300)
i i
Unfractionated system:
Copyright 2000 by CRC Press LLC
Mw
= 2, to Mw = 12, 000
Mn
23
Fractionated system: consider 1 mol of the system, then
Ni + N2 = 1.
∑ NiMi
= ∑ N i M i since ∑ N i = 1
∑ Ni
Mn =
= N i (2000) + N 2 (10, 000)
= 0.5 (2000) + 0.5 (10, 000)
= 6000
Note that Mn is the same for both the fractionated and unfractionated systems.
(
)
2
∑ N i M i 2 0.5 (2000) + 0.5 10, 000
Mw =
= 8667
=
∑ NiMi
6000
2
Melt viscosity is proportional to Mw. Since Mw (fractionated) < Mw (unfractionated), melt viscosity will
decrease.
Xn =
6.3a.
r=
Xn =
=
1+ r
2 r (1 − p) + 1 − r
100
= 0.95
105
1 + 0.95
2 (0.95) (1 − 0.98) + 1 − 0.95
1.95
= 22.16
0.088
Mn = XnM0
H
H2N
(CH2)5
COOH
N
O
(CH2)5
M o = 113
Mo ≡ 22.13 × 113 = 2504
b.
For maximum degree of polymerization, P = 1
Xn =
1 + r 1.95
=
= 39
1 − r 0.05
M n = 39 × 113 = 4407
Copyright 2000 by CRC Press LLC
C
24
POLYMER SCIENCE AND TECHNOLOGY
6.4a.
b.
Xn =
1
1− P
P
Xn
0.5
2
r=
0.99
100
1
∞
100
= 0.99
101
Maximum degree of polymerization
Xn =
r=
c.
Xn =
6.5. n HO
1 + r 1.99
=
= 199
1 − r 0.01
NA
100
100
=
=
= 0.96
N B + 2 N monoacid 100 + 2 × 2 104
1 + 0.96
= 49
1 − 0.96
O
O
C
C
O
O
C
C
O
OH + nHO
CH2CH2
CH2CH2
O + 2n H2O
n
a.
b.
Mn = Xn Mo =
Mo
1+ p
Mo =
192
= 96
2
Mn =
96
= 9600
1 − 0.99
1+ p
Mw = M o X w = M o
1− p
1 + 0.99 96 × 1.99
= 96
=
1 − 0.99
0.01
= 19,104
c.
Px = p x − p (1 − p)
= (0.99)
3−1
Copyright 2000 by CRC Press LLC
(1 − 0.99) = 9.80 × 10 −3
OH
25
d.
( )
Wx = X p x −1 (1 − p)
(
2
)
= 3 0.993−1 (1 − 0.99)
2
= 2.94 × 10 − 4
6.6.
(
)
Wx = X P x −1 (1 − p)
2
P
P
0.10
0.90
0.10
0.90
1
0.98
1
0.01
100
8.1 × 10–98
100
2.95 × 10–5
n
Wx
At low conversions, the fraction of monomer is high, while the fraction of higher molecular weight
polymers is low. Conversely, at high conversions the fraction of monomers is low, whereas the fraction
of high-molecular-weight polymers becomes much higher.
6.7.
Mn = Mo Xn =
Mo
1− p
For nylon 5,Mo = 113
324
= 162
For nylon 6,12 M o =
2
113
= 3260
Nylon 6, M n =
1 − 0.95
162
= 3240
Nylon 6,12, M n =
1 − 0.95
6.8.
N x = P x −1 (1 − P) = (0.98)
100 −1
2
(1 − 0.98)2 = 5.41 × 10 −5
Wx = XP x −1 (1 − P) = 100 (0.98)
2
100 −1
(1 − 0.98)2 = 5.41 × 10 −3
Fraction of monomers= 1(0.98)o (1 – 0.98)2
= 4.0 × 10–4
Copyright 2000 by CRC Press LLC
26
POLYMER SCIENCE AND TECHNOLOGY
SOLUTIONS TO CHAPTER 7 PROBLEMS
7.1.
Since f is independent of monomer concentration,
[M]
[I]1/2
Rp /[I]1/2[M]
9.04
8.63
7.19
6.13
4.96
4.75
4.22
4.17
3.26
2.07
0.485
0.454
0.0505
0.478
0.560
0.438
0.480
0.162
0.495
0.459
0.044
0.043
0.045
0.044
0.044
0.045
0.043
0.041
0.044
0.044
Since Rp /[I]1/2[M] is constant, data are consistent with rate of polymerization by free-radical mechanism.
fk
Rp = kp d
kt
7.2. a.
12
[I] 1 2 [M]
Assuming f is independent of [M] to keep Rp constant, [I] and [M] must be kept constant.
b.
d [M ⋅]
= 2 f k d [I]
dt
= 2 k d [I] assume f = 1
k d = 3.25 × 10 −4 min −1
= 5.42 × 10 −6 s−1
I = 6.6 × 10 −6 mol ml
Ri =
d [M ⋅]
= 2 5.42 × 10 −6 s−1 6.6 × 10 −6 mol ml
dt
(
)(
= 7.5 × 10 −11 mol ml − s
Xn =
Rp
Ri
R p = 1.79 × 10 −3 g ml − min
=
1.79
× 10 −3 mol ml − min
104
= 1.72 × 10 −5 mol ml − min
Xn =
1.72 × 10 −5 mol ml − min
4.29 × 10 −9 mol ml − min
= 4.01 × 103
Copyright 2000 by CRC Press LLC
)
27
− d [I]
= f k d [I]
dt
c.
− d [I]
= k d dt ; f = 1
[I]
[I] = e − k
=e
dt
(
− 3.25 × 10 −4 × 60 × 3
)
= 94.3%
7.3.
a.
In general, for radical chain polymerization, polymerization rates are given by
fk
Rp = kp d
kt
12
[I] 1 2 [M]
kd
→ 2R ⋅
I
= d [I] dt = fk d [I]
ln
[I]
[I ]
= kd t
if f = 1
o
kd =
ln 2
= 1.58 × 10 −2 h −1
44
= 4.38 × 10 −6 s−1
Substituting for the constants:
4.38 × 10 −6 s−1
R p = (145 l mol − s)
0.180 l mol − s
= 0.715[I]
12
b.
12
[I] 1 2 [M]
[M]
Assume that at 50% conversion:
[I] = [I o ]
[M] = [M o ]
2
Total volume = 100 + 400 + 0.5 = 500 ml = 0.51 since density = 1
[I] =
[M] =
0.5 g
1 mol
×
= 4.13 × 10 −3 mol l
0.5 l 242 gmol
50 g
1 mol
×
= 0.962 mol l
0.5 l 104 gmol
(
R p = 0.715 4.13 × 10 −3
= 0.044 mol l − s
Copyright 2000 by CRC Press LLC
)
12
(0.962) mol l − s
28
POLYMER SCIENCE AND TECHNOLOGY
k
R p = − d [M] dt = k p d
kt
c.
∫
k I
d [M]
= kp d o
[M]
kt
kp
[ ]
[ k [I ] k ]
12
d
o
[I ]
12
o
[M]
∫ dt
ln [M] M o
t=
7.4.
12
12
t
=
ln 2
[ ]
k I
kp d o
kt
12
= 15.1 s
The general expression for Xn when there is transfer is given by Equation 7.39 (in text book).
For the polymerization of pure styrene, there is obviously no solvent, hence the second term on the
right-hand side of the expression drops out.
a.
Comparing the remaining expression with the given equation:
C M = 0.60 × 10 − 4
b.
(
)
2
10 −4
1
10 −4
7
= 0.60 × 10 −4 + 8.4 × 10 2
2 + 2.4 × 10
Xn
(8.35)
(8.35)3
= 0.00166
X n = 602
c.
For normal termination:
[
R
1
2
= 8.4 × 10 2 p 2 = 8.4 × 10 2 10 −4 (8.35)
Xn
[M]
]
= 0.0012
X n = 833
d.
Comparing the given expression with the general transfer equation:
k p2 1.19
kt
i
e
=
=
× 10 −3 = 5.95 × 10 −4
840
,
.
.,
k p2
2k t
2
ν=
Copyright 2000 by CRC Press LLC
k p2
[M] 2
2k t
Rp
= 5.95 × 10 −4 (8.35) 10 −4 = 415
2
29
k
C I 2 t = 2.4 × 10 7
k p fk d
e.
C I = 2.4 × 10 7
k p2 fk d
kt
= 2.4 × 10 7 × 2.29 × 10 −9
= 0.055
k t k p2 = 840 ; f k d k p2 k t = 2.29 × 10 −9
f.
whence f k d = 1.92 × 10 −6
f=
7.5.
1.92 × 10 −6
= 0.61
3.2 × 10 −6
For transfer to solvent, the relevant equation is
1 X n = (1 X n )o + Cs [S] [M]
(1 X n )o = reciprocal degree of polymerization in the absence of
a solvent ( bulk polymerization)
M no = M m X no
where M m = mol wt of repeat unit
= 104 for PS
4.16 × 10 5 = 104 X no
X no = (4.16 104) × 10 5
1
= 2.5 × 10 −4
X no
Dilution factor = 2, i.e., [S]/[M] = 2
1 X no = 2.5 × 10 −4 + 2 Cs
Cyclohexane:
1 X n = 2.5 × 10 −4 + 2 × 0.16 × 10 −4
X n = 3550
M n = 3.69 × 10 5
Copyright 2000 by CRC Press LLC
30
POLYMER SCIENCE AND TECHNOLOGY
Carbon tetrachloride:
1 X n = 2.5 × 10 −4 + 2 × 180 × 10 −4
X n = 27.59
M n = 2.87 × 103
Mn =
7.6.
1
1
4
1
=
=
+ CS [S] [M]
M no , i.e.,
X n X no X no
4
CS [S] [M] = 6 × 10 −4
Styrene: [S]/[M] = 6 × 10–4/0.31 × 10–4 = 19.35
Methyl methacrylate: [S]/[M] = 6 × 10–4/0.52 × 10–4 = 11.54
Vinyl acetate: = [S]/[M] = 6/92 = 0.07
7.7.
We note first that high temperatures promote chain transfer and, consequently, a decrease in Xn. More
fundamentally, however, if termination predominates over transfer, then,
Xn ∝
kp
kt
∝e
(
− Ep −Et
) RT
Now if Et > Ep, then Xn should increase with decreasing temperature. In cationic polymerization, the
propagation step involves the approach of an ion to a neutral monomer molecule in a medium of low
dielectric constant. The activation energy required for this process is essentially zero. However, the
termination step requires the combination of an ion pair — a process that involves a considerable
activation energy. Thus in cationic polymerization, Et > Ep.
7.8.
Impurity
Water
Ethanol
Xn(Rp /Rt)
0.1
40
Thus the molecular weight of polystyrene formed in the presence of ethanol is 400 times that formed
in the presence of water. High polymer formation is more probable with ethanol impurity.
7.9.
Sterically regular structures of polymers are generally obtained only in the absence of free ions. If
anionic coordination is carried out in media of high solvating power like ethers or amines, the bond
between the metal of the catalyst and the carbon atom of the growing polymer chain is substantially
ionized. Consequently, the polymer anion behaves essentially as a free ion, and the positive counterion
is unable to perform its function of coordinating, orienting, and inserting the incoming monomer in the
polymer chain and therefore structural regularity cannot be assured. In effect, there is a change from
an anionic coordination polymerization mechanism to an anionic polymerization.
7.10. a. Kinetic chain length ν = average number of monomers consumed by each primary radical. In this
case ν is simply an equal distribution of available monomer among the initiators, that is,
ν = X n = [M] [I] = 1.0 1.0 × 10 −4
= 10 4
M n = M o X n = 104 × 10 4
Copyright 2000 by CRC Press LLC
= 1.04 × 10 6
31
b. For termination by coupling
M n = 2 X n M o = 2 ν = 2.08 × 10 6
c.
In the presence of transfer agent, Xn is reduced
X n = ν = [M]
([I] + [T])
(
= 1.0 1.0 × 10 −4 + 6.0 × 10 −4
)
= 1.43 × 103
M n = M o X n = 1.43 × 103 × 104
= 1.49 × 10 5
7.11. a.
fk
R p = − d [M] dt = k p d
kt
= K [I]
Run A, R p =
KA =
Run C, R p =
Kc =
12
12
[I] 1 2 [M]
[M]
1.0 × 0.5
mol l − min = 1.0 × 10 −3 mol l − min
500
Rp
[I] 1 2 [M]
=
1.0 × 10 −3
= 4.0 × 10 −2
(0.0025)1 2 (0.5 × 1.0)
0.8 × 0.40
= 5.33 × 10 −4 mol l − min
600
5.33 × 10 −4
= 3.5 × 10 −2
(0.001)1 2 (0.8 × 0.6)
Hence, K c ≈ K A = K = 4.0 × 10 −2
Run D, R p = 4.0 × 10 −2 (0.01)
12
(0.25 × 0.5)
= 5 × 10 −4 mol l − min
Time for this conversion =
0.5 × 0.25
5 × 10 −4
= 250 min
Copyright 2000 by CRC Press LLC
32
POLYMER SCIENCE AND TECHNOLOGY
ln R p60 R p80 =
b.
R p80 =
ln
E 1
1
−
R T60 T80
0.5 × 0.75
= 5.36 × 10 −4
700
E 1
1.0 × 10 −3
1
=
−
−4
5.36 × 10
1.99 333 353
(
0.624 = E 8.55 × 10 −5
E = 7.3 kcal mol
7.12.
Copyright 2000 by CRC Press LLC
)
33
SOLUTIONS TO CHAPTER 8 PROBLEMS
8.1.
From Table 8.1:
Monomer (1)
Monomer (2)
r1
r2
Methyl methacrylate
Styrene
Styrene
Vinyl chloride
0.46
17
0.52
0.02
Using the copolymer equation:
Feed
Composition,
f1
Methyl Methacrylate–Styrene
0.25
0.50
0.75
Styrene–Vinyl Chloride
F1
F2
F1
F2
0.31
0.49
0.67
0.69
0.51
0.33
0.86
0.96
0.98
0.14
0.06
0.02
For styrene–vinyl chloride copolymer, because of the large difference in the reactivity ratios of the
monomers, lowering the composition of styrene in the feed does not necessarily result in a reduction
of the styrene content in the copolymer. On the other hand, because of the similarity in reactivity ratios
in the styrene–methyl methacrylate system, the styrene content of the copolymer can be controlled by
an adequate control of feed composition.
F1 =
8.2.
r1 f12 + f1 f1 f2
r1 f12 + 2 f1 f2 + r2 f22
Hence, for r1 = 0.02 and r2 = 0.3, polymer composition is as follows:
f1
F1
0.25
0.50
0.75
0.34
0.44
0.49
r1 = k11 k12 = 0.02 or k11 = 0.02 k12
k 22 = 0.3 k 21
Each radical shows a greater preference for adding the other monomer. Consequently, each radical
prefers to add the other monomer, and as such there is a tendency toward alternation.
8.3.
For ideal copolymerization, the copolymer equation reduces to:
F1 =
r1 f1
r1 f1 + f2
r1 = 1 r2 = 0.1
F1 = 10 × 10.75 (10(0.75) + 0.25)
= 0.97
Copyright 2000 by CRC Press LLC
34
8.4.
POLYMER SCIENCE AND TECHNOLOGY
The possible monomer pairs and their corresponding r1r2 values are
Monomer Pair
Butadiene–styrene
Butadiene–acrylonitrile
Butadiene–vinyl chloride
Styrene–acrylonitrile
Acrylonitrile–vinyl chloride
r1r2
0.780
0.006
0.308
0.016
0.108
As the quantity r1r2 approaches zero, the tendency toward alternation increases. Therefore, butadiene–acrylonitrile will be the most suitable pair of monomers for the application under these conditions.
8.5.
Monomer
Structure
a. Acrylonitrile
CH2
CH
CN
b. Styrene
CH2
c. α-Methyl styrene
CH3
CH2
d. 2-Methyl styrene
e. Vinyl ethyl ether
CH
C
CH3
CH2
CH
CH2
CH
OEt
Order of reactivity: c > b > a > d > e.
Basis:
1.
General order of reactivity of monomers due to substituents:
−C6 H 5 > CN > OR
2.
3.
Additional substitution on the α-C is additive
2- or β-substitution reduces reactivity of monomer
Copyright 2000 by CRC Press LLC
35
8.6.
CH
CH + CH2
CO
CH
CH
CO
CH
CO
O
CH2
CH
CO
O
n
The tendency for alternation increases as the difference in polarity between two monomers increases.
Styrene is an electron donor monomer, while maleic anhydride is an electron acceptor. Consequently,
alternating copolymerization between both monomers is facilitated. Notice that r1r2 for the monomers
is 0.0006.
[
]
[
]
r1 = Q1 Q 2 exp − e1 (e1 − e 2 )
8.7.
r2 = Q 2 Q1 exp − e 2 (e 2 − e1 )
Monomer
1
A
A
C
2
r1
r2
r1 r2
F1
B
C
D
27.80
0.563
1.556
0.027
0.089
0.639
0.751
0.050
0.994
0.94
0.59
0.61
The copolymer from the polymerization of monomers A and C will show the highest alternation
tendency.
8.8.
Composition. From Equation 8.9:
Monomer Pair
F1 (%)
a
b
c
d
50.00
61.88
50.00
49.26
Sequence:
r1 = k11 k12 ; r2 = k 22 k 21
a.
b.
c.
d.
k11 = k12 = k22 = k21. The probability that monomer 1 will be next to monomer 2 in the chain is
exactly the same as either of the monomers being next to itself, and consequently the resulting
polymer is random. The composition of polymer formed later in the reaction remains unchanged.
k11 = 1.89 k12; k22 = 0.78 k11. In this case also, a random copolymer is formed, but there is a much
greater tendency for monomer 1 to be next to itself in the polymer monomer formed at the beginning
of the reaction. Later on in the polymerization, as a result of the depletion of monomer 1, the
polymer formed has a greater tendency of having monomer 2 next to itself.
k11 = 0.01 k12; k22 = 0.01 k21. Each monomer radical prefers the second monomer, and polymer
formed should be alternating. The composition and sequence of polymer at the initial and later
stages of the reaction are identical.
k11 = 0.002 k12; k22 = 0.032 k21. There is a strong tendency toward alternation. Polymer formed at
the later stages of reaction will be almost identical to that formed initially; however, polymer formed
later will contain more of monomer 1, while that formed initially will contain more monomer 2.
Copyright 2000 by CRC Press LLC
36
POLYMER SCIENCE AND TECHNOLOGY
SOLUTIONS TO CHAPTER 9 PROBLEMS
9.1. a. Better contact provides a better opportunity for electron transfer. Fast separation builds a high charge.
Slow separation allows some of the electrons that would have remained on the surface of the acceptor
to return to equilibrium, thus lowering the charge.
b. PVA/EVA offers a good balance of properties, including excellent outdoor weatherability, a complete
absence of plasticizer migration (especially important for medical uses) and improved low-temperature
capacity. These properties recommend PVA/EVA copolymer for outdoor and medical applications.
c. As long as the plasticizer is free of impurities, the choice of plasticizer has little or no effect on the
electrical properties of the PVC compound. Plasticizer permanence for long service life of wire and
cable formulations at increasingly high rating temperatures and flame retardancy are the most critical
criteria in selecting a plasticizer system. DUP, DTDP, and UDP, which have much higher molecular
weight, combined with the exceptionally high-temperature low volatility of trimellitates will more
readily satisfy the above criteria than DOP and DINP with relatively low molecular weight and
volatility.
d. One of the most important demands placed on PVC compounds for interior automotive uses is low
plasticizer volatility at high temperatures. A car interior can easily reach 82°C on a hot summer day
with all windows closed. A plasticizer that is relatively volatile at such temperature will cause fogging
of windshield and window surfaces. In such applications where nonfogging is important, the exceptionally low volatility of trimellitate plasticizers as well as their good processibility, compatibility,
and low water extraction make these plasticizers better alternatives than phthalate esters.
e. Any ingredient of a food container that can migrate into the food is subject to regulation. Substances
that are known to be carcinogenic or toxic are not sanctioned at all for food contact. Various regulations
relate to extraction of plasticizers from a food container by dry, oily, or aqueous foods. Plasticizers
from soya and linseed oil epoxides are derivatives from edible substances that are unlikely to be toxic.
The primary reasons for the popularity of adipates over phthalates in food-wrap films are their better
oxygen transmission, better resistance to water extraction, and better low-temperature flexibility.
f. Extraction of plasticizers from PVC by coatings such as nitrocellulose or the migration of the
plasticizers to the finishes soften the coatings making the surfaces unacceptable. The problem can
be reduced by the use of plasticizers such as polymerics with little or no affinity for the coating to
be encountered and the selection of a plasticizer with resistance to migration.
g. The polar nature of the C–Cl bonds promotes intermolecular interactions of the α-hydrogen of PVC
with carbonyl groups in various polyesters and, hence, miscibility with these systems. Polystyrene,
which has no carbonyl groups, is incapable of these interactions and is therefore completely immiscible with PVC. However, styrene–maleic anhydride copolymers contain both carbonyl groups and
the electron-withdrawing oxygen in the five-membered ring. These in-chain functionalities promote
intermolecular interactions that are responsible for the partial miscibility of PVC with SMA.
CH2
CH
CH2
CH
CH
CH
C
C
O
h.
O
(CH2)4
O
O
O
O
C
C
poly(butylene terephthalate) (PBT)
O
(CH2)2
O
O
O
C
C
poly(ethylene terephthalate) (PET)
Copyright 2000 by CRC Press LLC
O
37
Polycarbonate (PC), an amorphous polymer, provides the toughness for both blends. The crystallinity
of PBT and PET contributes the chemical resistance. The shorter methylene groups in PET relative
to PBT (2 vs. 4) enhance chain stiffness and hence high-temperature rigidity in PET.
i. Uninhibited ABS is subject to rapid deterioration of physical properties, which is characterized by
yellowing and embrittlement from thermal oxidation and outdoor exposure. This is due mainly to
the breakdown or oxidation of the unsaturated (double) bonds in the rubber (butadiene) phase. On
the other hand, oxidation of polystyrene homopolymer is relatively slow.
j. PVC is susceptible to thermal and oxidative degradation during processing and in service. Consequently, the polymer requires stabilization against the deteriorating effects of heat, light, and/or
oxygen. On the other hand, PVC has a high content of chlorine and therefore possesses inherently
some level of flame retardance. PVC, therefore, does not require the addition of flame retardants.
k. Black colors result in excessive heat build-up and are therefore avoided in such housing applications
as home siding and window profiles. Colorants containing heavy metals are avoided in toys and food
wrapping because of possible toxicity.
E L = E m (1 − φ f ) + E f φ f
9.2.
E f = 10.5 × 10 6 psi
(for E - glass)
E m = 2.0 × 10 5 psi
(for polypropylene)
Basis: 100 g of composite
Fiber content = 50 wt% = 50 g
lb 454 g 1 in.
Density of E-glass = 0.092
in.3 lb 2.54 cm
= 2.55 g cm 3
Vf =
50
= 19.61 cm 3
2.55
Vm =
50
= 55.25 cm 3
0.905
φf =
19.61
= 0.26
19.61 + 55.25
E L = 2.0 × 10 5 (1 − 0.26) + 10.5 × 10 6 (0.26)
= 2.88 × 10 6 psi
Copyright 2000 by CRC Press LLC
3
38
POLYMER SCIENCE AND TECHNOLOGY
SOLUTIONS TO CHAPTER 10 PROBLEMS
10.1. Both polyesters and nylon 6,6 are prepared by step-growth polymerizations. The activation energies for
such reactions are of the order of 84 kJ/mol. It is therefore usual to employ elevated temperatures to
accelerate these reactions. The step-growth polymerizations shown are characterized by polymerization–depolymerization equilibria, with equilibrium constants given by
Ke =
[ E] [ H2 O]
[COOH ] [OH ]
(1)
Ke =
[ N ] [ H2 O]
[COOH ] [OH ]
(2)
where [E] = concentration of the polyester; and [N] = concentration of nylon 6,6. From the definition
of the extent of reaction, p, and using Equation 3 for our discussion:
[E] = pCo and [COOH] = [OH] = C = Co (1 – p)
It follows that the equilibrium constant becomes
Ke =
[
] = p[ H O]
Co p H2O
C (1 − p)
2
o
2
Co (1 − p)
2
2
(3)
That is,
p HO
(1 − p)2 = [ 2 ]
(4)
Co Ke
To obtain a high-molecular-weight product, a high degree of conversion is imperative (i.e., p = 1). In this
case Equation 6 reduces to
[
]
HO
1− p = 2
Co Ke
12
(5)
Recall that the number average degree of polymerization is given by
Xn =
C K
1
= o e
1 − p H2O
[
]
12
(6)
It is obvious from Equation 6 that for a high value of Xn, the concentration of water in the reaction
mixture must be minimized. Since Ka @ Ke, the need to reduce the concentration of water in polyester
manufacture is more important in the manufacture of nylon 6,6. Consequently, the final stages of
polyester manufacture are carried out at reduced pressures.
10.2. For a first-order decay of the initiator,
[I] = [I o ] e − k t
d
Copyright 2000 by CRC Press LLC
39
In this case, Equation 10.12 becomes
ln
[M]
[M ]
2 k p f kd
kd kt
=
2
k e − kd t 2 − 1
kd
o
fk
where k = kp d
kt
12
=
[ I ] [e
12
kdt 2
o
[
]
−1
]
12
[Io]1/2 = 2.70 × 10–6 s–1 (Example 10.7).
2
2
=
s = 1.04 × 10 6 s
kd 1.92 × 10 −6
ln
[M]
[M ]
(
)(
= 1.04 × 10 6 s 2.70 × 10 −6 s −1
o
[M]
[M ]
= 0.9
o
[
ln 0.9 = 2.8 e −0.96 × 10
0.96 = e
−6
t
]
−1
( −0.96 × 10−6 t )
3.84 × 10 −2 = 0.96 × 10 −6 t
t = 4.0 × 10 4 s = 11.1 h
Copyright 2000 by CRC Press LLC
) [e
−0.96 × 10 −6 t
]
−1
40
POLYMER SCIENCE AND TECHNOLOGY
ln
[M]
[M ]
= − kt
o
fk
k = kp d
kt
k2 =
k p2
kt
12
[ I ]1 2
( f k ) [I]
d
l
−6 1
−3 mol
= 0.95 × 10 −3
1.92 × 10 4.0 × 10
s
mol − s
l
10.3.
= 7.30 × 10 −12 s−2
k = 2.70 × 10 −6 s−1
[M]
[M] o
ln
[M] o − [M]
[M ]
= −2.70 × 10 −6 t
= 0.8, i.e.,
o
[M]
[M ]
= 0.2
o
ln 0.2 = −2.70 × 10 −6 t
t = 5.96 × 10 5 s
Time required for 80% conversion is certainly unrealistic. However, it should be noted that Equation 10.2
holds only for the initial few percentage points of polymerization. At high conversions substantial
deviations occur — recall the Tromsdorff effect.
10.4. For a batch reactor
t=−
∫[
M
dM
Mo ] Rp
(1)
For thermal polymerization:
k
Rp = kp i
kt
12
12
[M] 2 = k [M] 2
k
where k = k p i . Substituting Equation 1 into Equation 2 and integrating yields
kt
Copyright 2000 by CRC Press LLC
(2)
41
kt =
k2 =
1
1
−
[M] [M] o
l
l
−11
⋅ ki = 8.5 × 10 −3
4.2 × 10
kt
mol − s
mol − s
k p2
l
= 35.7 × 10 −14
mol − s
2
k = 5.97 × 10 −7 l mol − s
l
s
k t = 2.5 h × 3600 5.97 × 10 −7
h
mol − s
= 0.0054 =
(3)
1
1
−
[M] [M] o
1
1 1.054
= 0.054 +
=
M
10
10
[M]
[M] o
=
9.49
= 94.9%
10
After 2.5 of thermal polymerization, residual monomer concentration is 94.9%, which means that the
product polymer cannot be processed into drinking cups without further purifications.
cal g mol °C
∆T = 18.2 × 103
g mol 29.5 cal
10.5.
= 617°C
10.6. The rate of polymerization for radical chain polymerization is given by the equation:
fk
Rp = kp i
kt
12
[I] 1 2 [M] ⋅
Assuming f is independent of both [I] and [M], then
R p = R [I]
k=
=
[I]
12
[M]
Rp
12
[M]
(1.94 × 10
(2.35 × 10
−4
−4
mol l − s
) (9 mol l)
mol l
= 1.41 × 10 −3 (l mol)
12
Copyright 2000 by CRC Press LLC
)
12
(s )
−1
42
POLYMER SCIENCE AND TECHNOLOGY
k=
=
[I]
Rp
[M]
12
(1.94 × 10
(2.35 × 10
−4
−4
mol l − s
)
) (9 mol
mol l
= 1.41 × 10 −3 (l mol)
12
12
(s )
−1
For solution polymerization
[M] =
9 mol
=2M
4.5 l
12
mol
l
R p = 1.41 × 10 −3
s −1 2.11 × 10 −4
mol
l
12
2 mol
l
= 1.41 × 1.45 × 2 × 10 −5 mol l− s
= 0.41 × 10 −4 mol l− s
Kinetic chain length υ =
Rp
Rt
=
Rp
Ri
1.94 × 10 −4
= 4.85
4.0 × 10 −4
For bulk polymerization υ b =
For solution polymerization υ s =
Ratio
(X )
(X )
n b
n s
=
0.41 × 10 −4
= 1.02
4.0 × 10 −4
4.85
= 4.75
1.02
10.7. Bulk polymerization is attractive for the production of polymers that are free from impurities from other
ingredients of the polymerization recipe. However, polymerization in batch reactors does not always
lead to 100% conversions. Consequently, the product polymer in this case must necessarily contain
unreacted monomers, which are normally difficult to remove. The use of a polymer with traces of a
toxic monomer for food wrap is therefore a bad decision.
10.8. The concentrations of micelles and droplets are about 1018 and 1011, respectively. In the recipe:
Volume of soap =
5
= 25 cm 3
0.2
Volume of monomer =
25
Vmic
=
= 0.2
Vdrop 125
Copyright 2000 by CRC Press LLC
100
= 125 cm 3
0.8
43
The surface-to-volume ratio of a sphere is 3/R.
S mic
V mic
S drop R drop
=
R mic
V drop
S mic V mic R drop
=
S drop V drop R mic
Average diameter of a micelle = 100 Å = 10–6 cm. Average diameter of a droplet = 10,000 Å = 10–4 cm.
S mic
10 −4
= 0.2 × −6 = 20
S drop
10
Ratio of total surface area of micelles to droplets:
N mic S mic
1018
⋅
= 20 × 11 = 20 × 10 7
N drop S drop
10
10.9a.
Rp = kp
N
[M]
2
N = 12.04 × 1014
= 2.00 × 10 −6
Rp =
1000 ml
particles
1 mol
ml 6.02 × 10 23 particles 1 l
mol
l
mol
1
l
−6 mol
5
550
2.00 × 10
mol − s
2
l
l
= 2750 × 10 −6
mol
= 2.75 × 10 −3 mol l s
l-s
At 80% conversion, monomer consumed = 0.8 × 5 M = 4
Time required to achieve this = 4
mol
l− s
1
−3
l 2.75 × 10 mol
= 1.45 × 10 3 s
= 0.40 h
Copyright 2000 by CRC Press LLC
mol
l
44
POLYMER SCIENCE AND TECHNOLOGY
Xn =
k p N[ M ]
Ri
1000 ml
radicals
1 mol
R i = 1.1 × 1012
23
mol − s 6.02 × 10 radicals 1 l
= 0.183 × 10 −8
mol
mol
= 1.83 × 10 −9
l− s
l− s
550 l
1 l− s
mol
−6 mol
5
Xn =
2.00 × 10
−9
l
l 1.83 × 10 mol
mol − s
= 3.01 × 10 6
= 3.01 × 10 6
b.
monomers
chain
monomer
chains
× 133
No. of chains per particle = 3.01 × 10 6
chain
particle
= 400 × 10 6
monomers
particle
monomers g mol monomers
= 400 × 10 6
particle 6.02 × 10 23 monomers
= 66.5 × 10 −17
g mol monomers
particle
86 g
−17 g mol monomer
66.50 × 10
particle
1.0 g mol monomer
Therefore, on the average, each particle weighs 5.72 × 10–14 g. But the average density = 1.25 g/cm3.
Copyright 2000 by CRC Press LLC
45
1 cm3
Average volume of particle = 5.72 × 10 −14 g
1.25 g
(
)
= 4.58 × 10 −14 cm 3
Vol of a sphere =
4 3
πR = 4.58 × 10 −14 cm 3
3
3
cm 3
4π
R 3 = 4.58 × 10 −14 ×
R 3 = 1.09 × 10 −14 cm 3
R = 2.22 cm × 10 −5 cm
D = 4.44 × 10 −5 cm
10.10. An energy balance for the isothermal reaction given:
(
)
V − ∆H p R p = UA(Tr − Tc )
V = πR 2 L = π
2
5.046
⋅ 2.5 = 50 m 3
2
V = reaction volume
U = overall heat transfer coefficient
A = wall heat transfer area
T = temperature of coolant
10 −4 mol
1l
1 cm 3
Rp =
3
l-s 1000 cm 10 −2 m
(
(
)
3
10 −1 mol
=
m 3 −s
)
kcal −1 mol
V − ∆H p R p = 50 m 3 21.3
10
mol
m 3s
= 106.49 kcal s
A w (T − Tc ) = 0.0135
= 0.0135
2
cal
4 cm
T − Tc )
π × 5.046 m × 2.5 m × 10
2 (
2
m
cm s K
(
= 5.35 × 10 3 (T − Tc )
T − Tc =
1.065 × 10 5
= 19.9
5.35 × 10 3
Tc = 50 − 19.9 = 30.1°C
Copyright 2000 by CRC Press LLC
)
cal
39.63 × 10 4 cm 2 (T − Tc )
cm 2 s K
46
POLYMER SCIENCE AND TECHNOLOGY
10.11. Consider a monomer mass balance
v[M] o − v [M] = Rp ⋅ V
v = volumetric flow rate
[M] o , [M] = monomer concentration in the feed and effluent, respectively
V = reactor volume
R p = rate of polymerization
[M] o − [M] = R p ⋅
V
= Rp τ
v
τ = mean residence time
Rp = kp
N
[M]
2
Conversion = 75% =
[ M ]o − [ M ]
[M ]
= 0.75
o
[M] = 0.25 [M o ]
[M] o − 0.25 M o
= kp
3 = kp
(
)
N
0.25[ M ] o ⋅ τ
2
N
⋅τ
2
6 = kp ⋅ N ⋅ τ
l 10 −3 m 3
k p = 2200
mol − s
l
= 2.20 m 3 mol − s
N = 1.8 × 1014
cm 3
1 mol
particles
3
23
6.02 × 10 particles 10 −6 m 3
cm
= 2.99 × 10 −4 mol m 3
τ=
6
1 mol − s 10 4 m 3
= 6
2.20 m 3 2.99 mol
kp ⋅ N
= 9.12 × 10 3 s
= 2.53 h
Flow rate:
V
= τ , V = 50 m 3
v
v=
V 50 m 3
=
τ 2.53 h
= 19.73 m 3 h
Copyright 2000 by CRC Press LLC
47
10.12a. Assuming first-order kinetics, then
M = M o e − kτ
Conversion p =
Mo − M Mo − Mo e − kτ
=
Mo
Mo
= 1 − e − kτ
τ = residence time
p = 1− e
(
)( ) = 1 − e −1 = 1 − 0.368
− 10 −2 10 2
= 0.632
b.
(
)
2
π 2 × 10 −2 L
V πR2 L πDL
= 10 2
=
=
=
4τ
4 × 3.14 × 10 −4
t
τ
L = 100 m
10.13. Adequate temperature control is a problem associated with bulk polymerization. This problem is
accentuated by high media viscosity. Chain reaction polymerizations are more exothermic than stepgrowth polymerizations. Also, the media viscosity in chain reaction polymerizations increases very
rapidly even at the early stage of the reaction. This contrasts sharply with step-growth polymerizations
where viscosity builds up gradually in the course of reaction. Proper mixing is a problem encountered
with the use of plug flow reactors and this can further enhance the problems of bulk polymerization.
Given the lower viscosity and milder temperatures in polycondensations, plug flow reactors will be
more favorable in the bulk polymerizations of poly(ethylene tetraphthalate) — a step-growth
polymer — than polyisobutylene — a chain reaction polymer.
10.14. Nylon 6,6 is a polycondensation polymer. To develop polymeric properties, step-growth polymers
require a high degree of conversion, which cannot be obtained in a CSTR. Putting a number of CSTRs
in series makes high extents of reaction possible.
10.15. Given the relative heats of polymerization of both monomers, solution polymerization will be more
appropriate for butene. This should permit a better temperature control, because the heat generated
can easily be dissipated by the choice of an appropriate solvent.
10.16.
Basis: 100 lb of polymer solution
Heat of polymerization = (10 lb)(620 Btu lb)
= 6200 Btu
Heat absorbed by solvent = mC p
= (90 lb) (0.96 Btu lb°F)
∆T =
6200
= 72°F
90 × 0.96
Feed temperature = 122 − 72 = 50°F
Copyright 2000 by CRC Press LLC
48
POLYMER SCIENCE AND TECHNOLOGY
SOLUTIONS TO CHAPTER 11 PROBLEMS
11.1. Vol of PVC material extruded per year = π (R c2 – R l2)L.
L=
20 ft 16 h 300 day 12 in.
×
×
×
h
day
year
ft
= 1.152 × 10 6 in. year
[
]
PVC vol year = π (2.5) − 2 2 × 1.152 × 10 6 in.3
2
= 8.143 × 10 6 in.3
Density = 1.44 g cm 3
1 lb
454 g
(2.54 cm)3
1 in.3
= 0.052 lb in.3
PVC mass year = 8.143 × 10 6 in.3 × 0.052 lb in.3
= 4.232 × 10 5 lb
Working hour year = 16 × 300 = 4800
PVC extrusion rate = 88 lb h = 90 lb h
Motor power requirement = 7 to 18 hp since 1 hp motor is required
for 5 to 10 lb h extruded material.
11.2.
a. The process of crystallization involves an orderly arrangement of polymer molecules from a disordered
polymer melt. Consequently, for a given mass of polymer, there is a decrease in volume as the
polymer crystallizes from the melt, i.e., crystallization results in shrinkage. Thus when a crystalline
polymer is extruded, the associated shrinkage as the polymer is cooled from the melt results in part
distortion. Under other hand, amorphous polymer when cooled from the melt undergoes a gradual
dimensional change and consequently it is easier to maintain the shape of the extruded part.
b. During melting of a crystalline polymer, additional heat is required to break up the intermolecular
bonding forces at the polymer melting point. In general, therefore, more heat is required to extrude
a crystalline polymer than an amorphous one. Therefore, if thermal conditions are comparable, the
extrusion rate is higher for amorphous than crystalline polymers.
c. Many polymers, depending on their composition, absorb moisture from the air. This can result in
steam formation and, consequently, frothing during extrusion. In some cases, the presence of water
promotes polymer degradation. The attendant molecular weight reduction can potentially change a
tough polymer into a brittle one. In either case, therefore, the presence of too much water in polymers
can result in poor product quality. It is, therefore, essential to remove water and volatiles from the
polymer before extrusion by installation of a hopper dehumidifier.
11.3.
hp = 0.0004 C pQ (Tm − Tf ) + 0.0004 QH f
Copyright 2000 by CRC Press LLC
49
where Hf = latent heat of fusion.
a.
Polystyrene
60 × 0.75 = 0.0004 × 0.45 (500 − 77) Q
Q=
b.
60 × 0.75
lb h = 591 lb h
0.0004 × 0.45 × 423
Polyethylene
0.75 × 60 = 0.0004 × 0.91 ( 440 − 77) Q + 0.0004 × 104 Q
= 0.1321 Q × 0.0416 Q
Q = 259 lb h
c.
Nylon 6,6
0.75 × 60 = 0.0004 × 0.63 (530 − 77) Q + 0.0004 × 5
45 = 0.1142 Q × 0.0224 Q
Q = 329 lb h
11.4.
Input drive power (in hp) =
Torque × rpm
5252
The torque a screw can safely carry is proportional to the cube of the root diameter. That is,
τ = kd3
For 2.5-in.-diameter screw,
τ = K (2.5) = 40 ×
3
K=
5252
200
5252 (1)
×
50
2.5
3
For 4.5-in.-diameter screw,
5252 (4.5)
×
50
2.5
3
150 rpm τ =
5252 (4.5)
150
×
×
= 176
50
2.5
5252
3
hp =
5252 (4.5)
200
×
= 233
50 2.5
5252
3
200 rpm hp =
Copyright 2000 by CRC Press LLC
50
POLYMER SCIENCE AND TECHNOLOGY
11.5. Volume of syringe =
πD o2 L
4
=
π (1) 6
4
= 4.71 in.3.
ε=
σ
; D = D o (1 + ε )
E
Volume of syringe due to dimensional change of material = πD2L/4.
Material
Modulus (psi)
∆V (in.3)
∆V (%)
Steel
Copper
Aluminum
30 × 106
18 × 106
10 × 106
4.77 – 4.71 = 0.06
4.81 – 4.71 = 0.10
4.88 – 4.71 = 0.17
1.3
2.1
3.6
11.6. High mold temperatures permit the polymer melt to cool more slowly and thus favor greater crystallinity.
This means that, on the average, a greater number of primary bonds must be broken for failure to occur
and therefore the molded part has a higher tensile strength. A higher degree of crystallinity also means
an increase in the possible difference between the refractive indices of the amorphous and crystalline
components (enhances optical anisotropy) of the molded part and consequently reduces part cavity.
11.7.
Extrusion Blow Molding
Polymer
Continuous
Intermittent
Poly(vinyl chloride)
Polyethylene
Poly(vinyl chloride), unlike polyethylene, is a heat-sensitive polymer. If used in the intermittent process,
it may suffer some degradation because polymer melt has to be kept hot in the extruder during the
blowing, cooling, and removal of part from the mold.
Copyright 2000 by CRC Press LLC
51
SOLUTIONS TO CHAPTER 12 PROBLEMS
12.1. Natural rubber is poly(cis-1,4-isoprene):
CH2
δ=
C
C
H
CH3
CH2
; M = 68
(2 × 131.5) + 121.5 + 84.5 + 148
ρΣ E
= 1.1
M
68
= 10.0
Polyacrylonitrile:
CH2
CH
; M = 53
CN
δ = 1.15
[131.5 + 86 + 354.5]
53
= 12.4
From Table 12.1:
Polymer
Most Suitable Solvent
Natural rubber
Polyacrylonitrile
Dichlorobenzene
Nitromethane
12.2.
Polymer
Poly(methyl methacrylate)
Poly(vinyl chloride)
Polyacrylonitrile
δ (cal/cm3)1/2
Solvent
δ (cal/cm3)1/2
9.25
9.9
12.35
Methyl methacrylate
Vinyl chloride
Acrylonitrile
Water
Dimethylformamide
8.8
7.8
11.9
23.4
12.1
a. In bulk polymerization a soluble initiator is added to pure monomer in the liquid state to effect
polymerization. In a homogeneous bulk polymerization system the resulting polymer is soluble in
the monomer, whereas in heterogeneous systems the polymer precipitates from the molten monomer.
The differences in solubility parameters between poly(methyl methacrylate) and methyl methacrylate
(about 0.45) and between poly(vinyl chloride) and vinyl chloride (about 2.1) suggest that PMMA is
soluble in its monomer whereas PVC is not.
b. In solution polymerization, both the starting monomer and resulting polymer are soluble in the solvent
medium. On the other hand, in precipitation polymerization the monomer is soluble, whereas the
polymer is insoluble in the solvent. Given the differences in their solubility parameters, both PAN
and acrylonitrile will be soluble in dimethyl formamide but certainly not in water.
12.3. When there is restriction to bond angles,
Copyright 2000 by CRC Press LLC
52
POLYMER SCIENCE AND TECHNOLOGY
r 2 = nl 2
1 − cos θ
1 + cos θ
θ = 109.5° ; cos θ = −0.334
r 2 = 2 nl 2
(r )
2
12
= 2n l
12.4
n = 2 D P = 2 × 10 6 72 = 2.78 × 10 4
a.
(
r02 = 670 Å
C ∞ = (670)
2
)
2
(2.78 × 10 )(1.53)
4
2
= 6.9
(
σ = r02 r02f
b.
)
12
r02 = 670 Å
1 − cos θ
r02f = nl 2
1 + cos θ
= 2 nl 2 since θ = 109.5°
(r )
2
0f
12
= 2 n l = 72 2 × 103 × 1.53
σ=
ap =
l
6.9 + 1
C ∞ + 1) =
1.53 = 6.04
(
2
2
1.
53
c.
670 72
= 1.86
2 × 103 × 1.53
109.5°
d.
32.25
r
(
)
r = nl coscos 32.25 = 2 × 10 6 72 × 1.53 cos 32.25
= 3.47 × 10 4 Å
Copyright 2000 by CRC Press LLC
53
12.5a.
At theta temperature
1 − cos θ 1 + cos θ
r02 = nl 2
1 + cos θ 1 − cos θ
=
2 M 2 1 − cos θ 1 + cos θ
l
M 0 1 + cos θ 1 − cos θ
where M is the molecular weight of the polymer, while M0 is the molecular weight of the repeating
unit. Assuming θ and φ are independent of molecular weight, which is generally true,
(r )
2
0
12
= k M1 2
b. Since the T = θ varies for different solvents and since solvent power increases with increasing
( )
temperature r02
12
will generally increase with temperature.
(r )
2
c.
12
( )
= α r02
12
= α k M1 2
12.6. From Equation 12.51:
Γ=
1 − χ1 =
=
ν22 M n
(1 − χ1 )
V1
Γ V1
Γ M1 ρ1
=
ν22 M n (1 ρ2 ) 2 M n
Γ ρ22 M1
ρ1 M n
(79.2 cm g)(1.19 g cm ) (72 g mol)
=
(0.8 g cm )(1.63 × 10 g mol)
3 2
3
3
5
= 0.062
χ1 = 0.5 − 0.062
= 0.438
From Equation 12.30:
1
ln a 1 = ln ν1 + 1 − ν 2 + χ1 ν 22
x
x=
Copyright 2000 by CRC Press LLC
V2 M 2 ρ2 1.63 × 10 5 1.19
=
=
= 1.52 × 10
V1
M1 ρ1
72 0.8
54
POLYMER SCIENCE AND TECHNOLOGY
(
)
ln a 1 = ln 0.5 + 1 − 1 1.52 × 10 3 0.5 + 0.438 (0.5)
2
a 1 = 0.92 = ρ1 ρ10
ρ1 = a 1 ρ10 = 0.92 × 100
= 92 mm Hg
12.7. Theoretically, for free rotation about valence bonds,
(r )
12
2
0
= 2n l
n = 2 DP = 2 M M o
where M0 = molecular weight of the monomer.
CH3
CH2
Polyisobutylene: Mo = 56
C
CH3
n
M o = 56
n=
(r )
12
2
0
r2
0
M
( r M)
( r M)
2
0
2
0
2M M
=
56
28
2 M
=
28
12
l
12
=
1.54
= 0.412
14
=
0.795
= 1.93 at 24°C
0.412
=
0.757
= 1.84 at 95°C
0.412
cal
12
exp
12
cal
a. Experimental values correspond to the dimensions of a real polymer where completely free rotation
about valence bonds is impossible due to steric hindrance.
b. As temperature increases, restrictions to free rotation decrease and, consequently, molecular size
decreases.
r2
M
12
= 0.795
2
1000
M=
= 1.58 × 10 6
0.795
Copyright 2000 by CRC Press LLC
55
r02f = 2 nl 2
12.8.
CH2
Mo = 104
CH
DP
n = 2 DP ; DP =
(r )
2
0f
1
2
10 6
= 9.62 × 10 3 , l − 1.54 Å
104
= 2n l
(
= 4 × 9.62 × 10 3
)
12
× 1.54 Å
= 302 Å
12.9a.
Contour length L = n (1.54 cos 35.25)
= 2 DP (1.54 cos 35.25)
=2×
8.85 × 10 5
(1.54 cos 32.25)
56
= 3.98 × 10 4 Å
b.
Volume of a molecule = Vm =
r3 =
=
M
4 3
πr =
3
ρ N Av
3M
4 π ρ N Av
3 × 8.85 × 10 5
4 π × 0.8 × 6.02 × 10 23
r = 76 Å
c.
( r M)
(r )
2
0
2
0
12
= 800 × 10 −11
12
= 800 × 10 −11 8.85 × 10 5
(
)(
)
12
= 750 Å
d.
Copyright 2000 by CRC Press LLC
Vrc 750 3
= 961; rc = random coil
=
Vcc 76
cc = compact coil
56
POLYMER SCIENCE AND TECHNOLOGY
In the solid state if random coils exist, there is interpenetration of molecules.
12.10.
natural rubber: cis-1,4-polyisoprene
CH3
H
C
C
CH2
CH2
gutta-percha: trans-1,4-polyisoprene
CH3
CH2
C
C
CH2
H
Repulsions between substituted groups are less in gutta-percha, where the isoprene units exist in trans
configuration, than in natural rubber with units in the cis configuration. Consequently, the hindrance
to coiled configurations as a result of steric interactions between near-neighboring units is less in guttapercha than in natural rubber. Therefore gutta-percha chains have unperturbed dimensions, more nearly
those which they would assume if all single-bond rotations were completely free.
ηsp = ηr − 1
12.11.
ηsp c = ( ηr − 1) c = [ η] + k ′ [ η] c
2
ηsp/C
C
0.618
0.625
0.702
0.744
0.829
0.946
0.275
0.344
0.896
1.199
1.604
2.108
Intercept of graph of ηsp/C vs. C from Figure S12.11A is 0.557 (dl/g). Therefore, [η] = 0.557 (dl/g).
2
dl
2
Slope of Figure 12.11A = 0.174 = k ′ [ η]
g
k ′ = Huggins constant =
0.174
(0.557)2
= 0.561
cm solvent
g dl
From Figure 12.11B, intercept = 1.482
RT
= intercept
Mn
Copyright 2000 by CRC Press LLC
57
Mn =
0.8205 × 1033 dl − cm solvent
RT
; R=
intercept
0.780
mol ⋅ K
Mn =
0.8205 × 1033 dl − cm − solvent
1
(303 K)
0.780
mol ⋅ K
1.482 cm solvent
= 2.22 × 10 5 g mol
RTA 2 = slope
A 2 = slope RT
cm − solvent
mol
0.780
= 0.636
2
0.8205 × 1033 × 303 dl ⋅ cm solvent
(g dl)
= 1.93 × 10 −6
dl − mol
g2
1
π
= RT
+ A 2 c + A3 c2 + L
c
Mn
12.12a.
Solution A:
From Figure S12.12,
RT
π
as c → 0
= intercept on
c
Mn
= 158.03
cm solvent
g cm 3
82.06 cm 3 × 1033 cm H 2O
1
Mn =
(300 K)
cm
H
mol − K
2
158.03 × 0.85
3
g cm
Note: cm solution ≈ cm solvent
= cm H 2O ρs
where rs = density of solvent
Mn = 1.89 × 105 g/mol
Solution B:
Intercept = 296.77
cm solution
g cm 3
82.06 cm 3 × 1033 cm H 2O
1
Mn =
(300 K)
cm
H
O
mol − K
2
296.77 × 0.85
3
g
cm
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58
POLYMER SCIENCE AND TECHNOLOGY
= 1.01 × 10 5 g mol
Slope = RTA 2
Solution A: slope = 17059 cm solution/(g/cm3)
A2 =
1705 g cm solution g cm 3
82.06 cm 3− atm mol − K (300 K)
(
)
= 5.7 × 10 −4 mol − cm 3 g2
Solution B: slope = 2373/cm solution/(g/cm3)
A2 =
(
23731 × 0.85 cm H 2O g cm 3
)
82.06 cm 3 ⋅ cm H 2 O mol − K × 1033 (300 K)
= 7.10 × 10 −4 mol − cm 3 g2
b.
Basis = 100 g mixture
Mn =
Σ wi
=
Σ wi Mi
100
25
75
+
1.89 × 10 5 1.01 × 10 5
= 1.14 × 10 5
c.
Solution A:
Mw = 2 × 1.89 × 10 5 = 3.78 × 10 5
Solution B:
Mw = 2 × 1.01 × 10 5 = 2.02 × 10 5
Mw =
∑ w M = 0.25 × 3.78 × 10
i
5
i
= 2.46 × 10 5
Mw 2.46 × 10 5
=
M n 1.14 × 10 5
= 2.16
Copyright 2000 by CRC Press LLC
+ 0.75 × 2.02 × 10 5
59
SOLUTIONS TO CHAPTER 13 PROBLEMS
13.1. Resilience is the area under the elastic portion of the stress–strain curve.
Re silience =
∫
σy
σdε
0
σ = Eε
Resilience =
∫
εy
(Eε )
Eεdε =
2
y
2
0
εy =
σy
E
σ y2
E σy
=
2 E
2E
2
Resilience =
(62.05 × 10 ) (N m ) = 79.78 × 10
=
2 (24 13 × 10 )( N m )
6 2
2
3
2
4
J
13.2.
Polymer
% Reduction of area
T
S
(2.0 – 1.0)/2.0 = 50
(20.0 – 18.0)/20 = 10
Children’s toys need to withstand lots of abuse without breaking (i.e., should be tough). Therefore
polymer T would be more suited for this application.
13.3. Nylon 6,6 with its strongly polar hydrogen bonding and polycarbonate with combined strong polar
forces and rigid backbone show high rigidity and strength combined with a good measure of ductility.
13.4. At σN , the load is maximum.
F = σA
dF = σdA + Adσ = 0
i.e.,
dσ/σ = –dA/A
For uniform deformation, there is a constancy of the volume of the material being deformed, i.e.,
AL = A 0 L 0
A = (A 0 L 0 ) L
dA = − A 0 L 0 dL L2 = − AL dL L2 = − AdL L
or
− dA A = dL L
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60
POLYMER SCIENCE AND TECHNOLOGY
but
dε = dL L
dσ σ = − dA A = dL L = dε
dσ dε = σ
For the relation
σ = Kε n
dσ dε = n Kε n −1 = σ
i.e., nKεn–1 = Kεn. This can only be true if ε = n.
σ n = Kε n
= K εε
= 10 6 (0.5)
0.5
N m2
= 7.07 × 10 5 N m 2
13.5.
(σ )
s max
=σ 2
σ = 2(σ s )max
σ = P A = 2(σ s )max
P = 2 A(σ s )max
(
)(
= 2 × 10 −4 m 2 10 6 N m 2
= 2 × 10 2 N
Copyright 2000 by CRC Press LLC
)
61
13.6.
Slip plane = (110)
Normal to slip plane = [110]
Slip direction [111]
Tensile axis [100]
φ = angle between tensile axis and normal to the slip plane
θ = angle between tensile axis and slip direction
a = vector along tensile axis = i
b = vector along slip direction = i+j+k
c = vector normal to slip plane = i+j
r r
a⋅b
1
cos θ = r r =
3
d b
rr
a⋅c
1
cos θ = r r =
2
d c
(σ )
s max
= σ cos θ cos φ
(
)(
=σ1 3 1 2
σ = (σ s ) max
)
6
= 27.58 × 10 6
6 N m2
= 67.55 × 10 6 N m 2
Copyright 2000 by CRC Press LLC
62
POLYMER SCIENCE AND TECHNOLOGY
13.7. a. The fact that materials from the copolymer are transparent in spite of the differing refractive indices
of the respective homopolymer suggests that the SBS polymer is a block copolymer existing in two
separate phases. From Figure 13.25, room temperature lies between the glass transition temperatures
of polybutadiene (–80°C) and polystyrene (110°C). At room temperature the rubbery polybutadiene
domains are cross-linked by the hard and glossy polystyrene regions. Consequently, the tensile
strength reflects the state of the polystyrene regions and remains high.
b. Natural rubber is soluble in hexane, while PMMA is soluble in ethyl acetate. Therefore, when the
graft copolymer is isolated from ethyl acetate, the rubber chains assume coiled conformation or
collapsed, while the PMMA chains are extended. The resulting physical properties of the copolymer
resemble those of PMMA. On the other hand, when the copolymer is isolated from hexane, the
PMMA chains are coiled while the rubber chains are extended. The properties in this case resemble
those of rubber.
c. In water, the hydrophilic poly(vinyl alcohol) blocks are solubilized and extended holding the tightly
coiled polystyrene segments in solution. In benzene, the opposite situation occurs, but in acetone,
with intermediate properties between water and benzene, both blocks are relatively extended.
d. Copolymers of hexamethylene adipamide and hexamethylene terephthalamide are isomorphous,
while copolymers of hexamethylene sebacamide and hexamethylene terephthalamide are nonisomorphous. The addition of a comonomer to a crystalline polymer results in a drastic loss in crystallinity
and the attendant properties, except in cases where both comonomers form isomorphous copolymers.
13.8.
Copyright 2000 by CRC Press LLC
63
13.9.
Polymer
Tg °C
Structure
-13
Poly(vinyl methyl ether)
CH3
CH3
105
Poly(vinyl formal)
Poly(ethyl methacrylate)
100
CH3
CH2
C
C
O
O
C
CH2
CH3
Poly(t-butyl methacrylate)
135
CH3
CH2
C
C
O
O
CH3
C
CH3
CH3
In the first pair of polymers, the introduction of rings into the main chain of poly(vinyl formal) increases
the Tg significantly compared with poly(vinyl methyl ether) with relatively flexible side chains. In the
second pair of polymers, the presence of the much bulkier t-butyl group in the ester side group enhances
chain stiffness and therefore raises the Tg.
13.10. As n increases, the side chain becomes flexible, and the overall stiffness of the polymer decreases.
13.11. a. Plane stress:
E Gc
σf =
πa
12
b. Plane strain:
EG
c
σf =
2
πa 1 − ν
(
Copyright 2000 by CRC Press LLC
)
12
64
POLYMER SCIENCE AND TECHNOLOGY
In this case, σf = σu, hence for plane stress for material H:
E Gc
a=
=
π σ f2
(532 × 10
)
N-m
N m 2 0.06 × 10 3
m2
6
2 2
(3.14) 15.3 × 10 N m
6
(
)
= 43.4 × 10 −6 m = 43.4 Å
a x 10-6 (m)
Material
Plane Stress
Plane Strain
S
H
2136
43
213600
4340
Material S, unlike H, is rubbery and therefore capable of extensive plastic deformation. Consequently,
only very large cracks can cause rupture. Material S shows extensive deformation before fracture
while H is relatively brittle. Thus S will be a preferable material in the application.
13.12. From the dynamic mechanical properties, resin D is evidently more flexible than resin A. As the resins
are cured, the more flexible resin (D) develops less internal stress than the stiffer resin A due to greater
molecular mobility. As wood absorbs and releases moisture during cyclic wet–dry treatment, the
adhesive joint is subjected to cyclic stress. Resin D, due to its flexibility, is able to respond reversibly
to the cyclic stress whereas the stiffer resins begin to degrade after five wet–dry cycles.
13.13. Chains containing either linkages are expected to be relatively flexible. As cure time increases the
number of these chains formed apparently increases also, and consequently the fracture initiation
energy increases due to increased ability for plastic deformation. However, at the onset of decomposition of these chains into the more rigid methylene bridges (1) or scission products (2 and 3), a drop
in the fracture energy would be predicted.
The difference between the fracture initiation and arrest energies (GIc – GIa) is the energy released
during crack propagation. If GIc – GIa is small, it implies (in most cases) that the material in the
immediate vicinity of the crack tip undergoes plastic deformation, blunts the crack, and arrests its
propagation. In cases where GIc is large, a propagating crack travels long distances before being
arrested. Therefore, the magnitude of GIc – GIa is a measure of a materials resistance to catastrophic
failure. Obviously, material B is more cured than material A; GIc – GIa is considerably larger for
material B than A and consequently, material B is more susceptible to catastrophic failure.
OH
OH
CH2
13.3.
+
OH
OH
CH2
O
CH2O
(1)
OH
OH
CHO + H3C
CH2
(2)
Methylene ether linkages formed
from initial cure reactions
O
CH2 + H2O
2
Possible subsequent decomposition
products on further cure
Copyright 2000 by CRC Press LLC
(3)
65
SOLUTIONS TO CHAPTER 14 PROBLEMS
14.1. a. The strains of the spring and the Voigt element are additive. Therefore, the rheological equation for
the model is
ε = ε1 + ε 2
=
σ0 σ0
1 − e − t τ2
+
E1 E 2
[
]
where t2 = η2/E2 .
b.
14.2. For the generalized Maxwell model, the relaxation modulus is given by
E r (t ) =
n
∑E e
− t τi
i
i =1
= 10 5 e −10 10 + 10 6 e −10 / 20 + 10 7 e −10 30
= 78.04 × 10 5 N m 2
14.3.
Copyright 2000 by CRC Press LLC
J (t ) =
ε (t )
=
σ0
4
∑ J (1 − e )
− t τi
i
i =1
Element No.
Ji(mN)
ti (s)
1
2
3
4
0.2 × 10
10–10
0.2 × 10–8
10–8
100
5
10
500
–8
66
POLYMER SCIENCE AND TECHNOLOGY
(
(
)
)
(
)
(
J (t ) = 0.2 × 10 −8 1 − e −100 100 + 10 −10 1 − e −100 5 + 0.2 × 10 −8 1 − e −100 10 + 10 −8 1 − e100 50
)
= 0.126 × 10 −8 + 10 −10 + 0.2 × 10 −8 + 10 −8 × 0.181
= 0.517 × 10 −8
(
)( )
ε (t ) = J (t ) × σ 0 = 0.517 × 10 −8 108
ε (t ) = 0.517
14.4. For this material the creep compliance from the Boltzmann superposition principle is
ε (t ) = J (10, 000) σ 0 + J (10, 000 − 100) 10 7 + J (10, 000 − 1000) 10 6
[
−4 (10000 −1000 )
−10 −4 (10 4 −100 )
−10 4 (10 −4 )
−8
8
10
10
1
e
10 7 + 10 −8 1 − e −10
= 10 −8 1 − e
+
−
= 0.632 + 0.63 + 0.006
= 0.701
14.5. First, find the time at Tg:
log10 a t = log
=
−17.44 [0 − (−70)]
tT
=
t Tg
51.6 + T − Tg
−17.44 × 70
= +10.04
51.6 + 70
t 0° C
= 9.13 × 10 −11 h
t −70°C
10 4
= t −70°C
9.03 × 10 −11
t −70°C = 1.095 × 1014 h
Now use this value to determine the T.
log
log
tT
−17.44 (T + 70)
=
t −70°C 51.6 + t −70°C + 70
−17.44 (T + 70)
10
= log10 9.13 × 10 −14 =
1.095 × 1014
51.6 + T + 70
−13.04 =
−17.44 (T + 70)
121.6 + T
T = 83°C
Copyright 2000 by CRC Press LLC
]
67
14.6.
τ oc
log
log
(
)
(
)
−17.44 T − Tg
τT
=
51.6 + T − Tg
τ Tg
log
τ Tg
=
−17.44 O − Tg
51.6 + O = Tg
+17.44 Tg
10 4
=
13
10
51.6 − Tg
Tg = −55°C
log
τ 25
−17.44 × 80
=
= −10.6
τ −55
131.6
τ 25 = 2.5 × 10 2 s
log a T = log
log
ω Tg
ωT
ω Tg −17.44 (130 − 150)
tr
= log
=
51.6 + 130 − 150
ωT
t Tg
= 11.04
ω Tg = 1.09 × 1011 ω T
= 1.09 × 1011 (ω T = 1 cycle s)
14.7.
log
log
ω Tg
ωT
=
−17.44 (T − 150)
T − 98.4
1.09 × 1011 −17.44 (T − 150)
=
10 3
T − 98.4
8.04 =
−17.44 (T − 150)
T − 98.4
τ = 194°C
log a T = log
14.8.
τT
−17.44 (T − 100)
=
τ Tg 51.6 + 150 − 100
For T = 150°C
log
τ150 −17.44 (150 − 100)
=
τ100
51.6 + 150 − 100
= −8.58
( )
τ150 = 2.61 × 10 −9 s τ Tg
Copyright 2000 by CRC Press LLC
68
POLYMER SCIENCE AND TECHNOLOGY
For T = 125°C
log
τ125 −17.44 (125 − 100)
=
τ100
51.6 + 125 − 100
= 5.69
τ125 = 2.03 × 10 −6
τ150 2.61 × 10 −9
=
τ125
2.03 × 10 6
= 1.29 × 10 −3
Copyright 2000 by CRC Press LLC
69
SOLUTIONS FOR CHAPTER 15 PROBLEMS
15.1. a. Copolymerization with propylene helps to reduce the density of HDPE. HDPE is used for telephone
cable insulation because of its greater toughness and because it experiences less absorption of
petroleum jelly and other organic cable fillers that are placed into telephone cables to exclude water.
Improved toughness helps to resist damage when individual insulated wires are assembled into cables
using high-speed machinery. An extremely broad MWD is needed to achieve good extrusion characteristics at the high wire speeds involved.
b. The high crystalline melting point of polypropylene (Tm = 165 to 171°C) along with its high heat
deflection temperature permits products made from polypropylene to be heat sterilized. On the other
hand, the relatively low crystalline melting point of polyethylene and, particularly, the low heat
deflection temperature of PE and PS limits the range of good mechanical properties and, as such,
polyethylene and polystyrene products are susceptible to deterioration in quality due to heat sterilization.
c. Polypropylene is a nonpolar hydrocarbon polymer. It is therefore not affected by polar solvents like
ethanol and acetone. However, it has a tendency to swell in nonpolar solvents such as benzene and
carbon tetrachloride.
d. Since semicrystalline polymers are generally not 100% crystalline, they have regions of differing
refractive indices; that is, they exhibit optical anisotropy. Consequently, semicrystalline polymers
like polypropylene and polyethylene — particularly in thick sections — are translucent. On the other
hand, amorphous polymers are optically isotropic and hence transparent. Random copolymerization
of propylene and ethylene results in mutual destruction of crystallinity. The resulting random copolymer is therefore transparent. Polymers for film application must necessarily be tough and flexible.
This is typical of random copolymers of propylene and ethylene. Filaments must have high modulus
and tensile strength. These requirements are satisfied by polypropylene homopolymer.
e. Chlorinated PVC has a higher heat deflection temperature than pure PVC. Consequently, it is more
suitable for use in piping for hot-water systems.
f. Manufacture of PVC involves predominantly suspension, emulsion, and, sometimes, bulk polymerization of vinyl chloride. In all cases, the reaction does not usually go to completion. Unreacted
monomer must be removed from the final product. Commercial PVC may therefore be expected to
contain a certain amount of vinyl chloride. Since vinyl chloride is a suspected carcinogen, its contact
with food must generally be avoided. This limits the use of PVC in food packaging. Besides, PVC
used for food packaging must meet stringent standards to prevent possible contact of food with any
unreacted vinyl chloride, a potential carcinogen.
g. Cold temperature toughness, gloss, and chemical resistance permit general-purpose ABS to be used
in applications requiring refrigeration. Appliance housings, business machines, television cabinets,
and interior aircraft uses are applications that require increased resistance to ignition from minor
flame sources, hence the use of fire-retardant ABS grade.
h. Because of the pressure of polar amide groups and the effect of absorbed moisture on dielectric
properties, nylon insulation is restricted by low-frequency applications. Nylon toughness and abrasion
resistance make it suitable as a protective covering for primary electrical insulation.
i. Compared with nylons 6 and 6,6, nylons 11, 12, 6,12 have much lower moisture absorption, possess
increased flexibility at high and low temperatures, and have exceptional resistance to stress cracking.
j. PPS has excellent chemical resistance and thermal resistance and is inherently flame retardant. Its
chemical resistance is exemplified by its extreme inertness to organic solvents and inorganic salts
and bases. This, combined with its hardness and nonstick characteristics, makes PPS a corrosionresistant material suitable for food contact and aggressive environments.
k. Poly(chlorotrifluoroethylene) chains have a much lower symmetry than polytetrafluoroethylene due
to the presence of the relatively larger chlorine. Consequently, the degree of molecular order in
poly(chlorotrifluoroethylene), and hence crystallinity, is relatively lower than in PTFE. This is
reflected in the relative melting points. Similarly, the lower symmetry in the chlorine-containing
polymer prevents crystallization when the polymer is quenched, thus resulting in a completely
amorphous polymer that is generally optically clear. The presence of chlorine makes poly(chlorotrifluoroethylene) a polar polymer. This detracts from its electrical properties.
Copyright 2000 by CRC Press LLC
70
POLYMER SCIENCE AND TECHNOLOGY
l. The presence of double bonds in the skeletal structure of a polymer makes it susceptible to oxidative
degradation and attack by ozone. Unlike most elastomers, butyl rubber has virtually no unsaturation
in its skeletal structure.
Copyright 2000 by CRC Press LLC